Php Strings

Introduction
A string is series of characters(mostly alphanumeric), where a character is the same as a byte. This means that PHP only supports a 256-character set, and hence does not offer native Unicode support.

Note: It is no problem for a string to become very large. PHP imposes no boundary on the size of a string; the only limit is the available memory of the resident computer on which PHP is running. Syntax

A string literal can be specified in four different ways:
 * 1) single quoted
 * 2) double quoted
 * 3) heredoc syntax
 * 4) nowdoc syntax (since PHP 5.3.0)

Single quoted
This is the simplest way to specify a string by to enclosing it in single quotes (the character '). To specify a literal single quote, escape it with a backslash (\). To specify a literal backslash, double it (\\). All other instances of backslash will be treated as a literal backslash: this means that the other escape sequences you might be used to, such as \r or \n, will be output literally as specified rather than having any special meaning.

Note: Unlike the double-quoted and heredoc syntaxes, variables and escape sequences for special characters will not be expanded when they occur in single quoted strings. <?php echo 'this is a simple string'; echo 'You can also have embedded newlines in                  strings this way as it is               okay to do'; // Outputs: Arnold once said: "I'll be back" echo 'Arnold once said: "I\'ll be back"';

// Outputs: You deleted C:\*.*? echo 'You deleted C:\\*.*?'; // Outputs: You deleted C:\*.*? echo 'You deleted C:\*.*?'; // Outputs: This will not expand: \n a newline echo 'This will not expand: \n a newline'; // Outputs: Variables do not $expand $either echo 'Variables do not $expand $either'; ?>

Double quoted
If the string is enclosed in double-quotes ("), PHP will interpret more escape sequences for special characters: Escaped charactersSequence	Meaning \n	                       linefeed (LF or 0x0A (10) in ASCII) \r	                        carriage return (CR or 0x0D (13) in ASCII) \t	                        horizontal tab (HT or 0x09 (9) in ASCII) \v	                        vertical tab (VT or 0x0B (11) in ASCII) (since PHP 5.2.5) \f	                        form feed (FF or 0x0C (12) in ASCII) (since PHP 5.2.5) \\	                        backslash \$	                        dollar sign \"	                        double-quote \[0-7]{1,3}	               the sequence of characters matching the regular expression is a character in octal notation \x[0-9A-Fa-f]{1,2}	       the sequence of characters matching the regular expression is a character in hexadecimal notation

As in single quoted strings, escaping any other character will result in the backslash being also output too.

The most important feature of double-quoted strings is the fact that variable names will be expanded.

Heredoc
A third way to delimit strings is the heredoc syntax: <<<. After this operator, an identifier is provided, then a newline. The string itself follows, and then the same identifier again to close the quotation. The closing identifier must begin in the first column of the line. Also, the identifier must follow the same naming rules as any other label in PHP: it must contain only alphanumeric characters and underscores, and must start with a non-digit character or underscore. Warning: It is very important to note that the line with the closing identifier must contain no other characters, except possibly a semicolon. That means especially that the identifier may not be indented, and there may not be any spaces or tabs before or after the semicolon. It's also important to realize that the first character before the closing identifier must be a newline as defined by the local operating system. This is \n on UNIX systems, including Mac OS X. The closing delimiter (possibly followed by a semicolon) must also be followed by a newline. If this rule is broken and the closing identifier is not "clean", it will not be considered a closing identifier, and PHP will continue looking for one. If a proper closing identifier is not found before the end of the current file, a parse error will result at the last line. Heredocs can not be used for initializing class properties. Since PHP 5.3, this limitation is valid only for heredocs containing variables.

Example (wrong example) 

Heredoc text behaves just like a double-quoted string, without the double quotes. This means that quotes in a heredoc does not need to be escaped, but the escape codes listed above can still be used. Variables are expanded, but the same care must be taken when expressing complex variables inside a heredoc as with strings.

Example string quoting example foo = 'Foo'; $this->bar = array('Bar1', 'Bar2', 'Bar3'); } } $foo = new foo; $name = 'MyName'; echo <<foo. Now, I am printing some {$foo->bar[1]}. This should print a capital 'A': \x41 EOT; ?>

The above example will output: My name is "MyName". I am printing some Foo. Now, I am printing some Bar2. This should print a capital 'A': A

It is also possible to use the Heredoc syntax to pass data to function arguments:

Example Heredoc in arguments example 

As of PHP 5.3.0, it's possible to initialize static variables and class properties/constants using the Heredoc syntax:

Example #4 Using Heredoc to initialize static values 

Starting with PHP 5.3.0, the opening Heredoc identifier may optionally be enclosed in double quotes:

Example #5 Using double quotes in Heredoc 

Nowdoc
Nowdocs are to single-quoted strings what heredocs are to double-quoted strings. A nowdoc is specified similarly to a heredoc, but no parsing is done inside a nowdoc. The construct is ideal for embedding PHP code or other large blocks of text without the need for escaping. It shares some features in common with the SGML <![CDATA[ ]]> construct, in that it declares a block of text which is not for parsing.

A nowdoc is identified with the same <<< sequence used for heredocs, but the identifier which follows is enclosed in single quotes, e.g. <<<'EOT'. All the rules for heredoc identifiers also apply to nowdoc identifiers, especially those regarding the appearance of the closing identifier.

Example #6 Nowdoc string quoting example foo = 'Foo'; $this->bar = array('Bar1', 'Bar2', 'Bar3'); } } $foo = new foo; $name = 'MyName'; echo <<<'EOT' My name is "$name". I am printing some $foo->foo. Now, I am printing some {$foo->bar[1]}. This should not print a capital 'A': \x41 EOT; ?>

The above example will output: My name is "$name". I am printing some $foo->foo. Now, I am printing some {$foo->bar[1]}. This should not print a capital 'A': \x41

Note:

Unlike heredocs, nowdocs can be used in any static data context. The typical example is initializing class properties or constants:

Example Static data example 

Note:

Nowdoc support was added in PHP 5.3.0.

Variable parsing

When a string is specified in double quotes or with heredoc, variables are parsed within it.

There are two types of syntax: a simple one and a complex one. The simple syntax is the most common and convenient. It provides a way to embed a variable, an array value, or an object property in a string with a minimum of effort.

The complex syntax was introduced in PHP 4, and can be recognised by the curly braces surrounding the expression. Simple syntax

If a dollar sign ($) is encountered, the parser will greedily take as many tokens as possible to form a valid variable name. Enclose the variable name in curly braces to explicitly specify the end of the name. 

The above example will output: He ate some cake snack. He ate some snack made of.

Similarly, an array index or an object property can be parsed. With array indices, the closing square bracket (]) marks the end of the index. The same rules apply to object properties as to simple variables.

Example:  "purple"); echo "He drank some $juices[0] juice.".PHP_EOL; echo "He drank some $juices[1] juice.".PHP_EOL; echo "He drank some juice made of $juice[0]s.".PHP_EOL; // Won't work echo "He drank some $juices[koolaid1] juice.".PHP_EOL; class people { public $john = "John Smith"; public $jane = "Jane Smith"; public $robert = "Robert Paulsen"; public $smith = "Smith"; } $people = new people; echo "$people->Jane drank some $juices[0] juice.".PHP_EOL; echo "$people->Jane then said hello to $people->jane.".PHP_EOL; echo "$people->Jane's wife greeted $people->robert.".PHP_EOL; echo "$people->Josepn greeted the two $people->smiths."; // Won't work ?>

For anything more complex, you should use the complex syntax. Complex (curly) syntax

This isn't called complex because the syntax is complex, but because it allows for the use of complex expressions.

Any scalar variable, array element or object property with a string representation can be included via this syntax. Simply write the expression the same way as it would appear outside the string, and then wrap it in { and }. Since { can not be escaped, this syntax will only be recognised when the $ immediately follows the {. Use {\$ to get a literal {$. Some examples to make it clear: width}00 centimeters broad.";

// Works, quoted keys only work using the curly brace syntax echo "This works: {$arr['key']}";

// Works echo "This works: {$arr[4][3]}";

// This is wrong for the same reason as $foo[bar] is wrong outside a string. // In other words, it will still work, but only because PHP first looks for a // constant named foo; an error of level E_NOTICE (undefined constant) will be // thrown. echo "This is wrong: {$arr[foo][3]}";

// Works. When using multi-dimensional arrays, always use braces around arrays // when inside of strings echo "This works: {$arr['foo'][3]}";

// Works. echo "This works: ". $arr['foo'][3];

echo "This works too: {$obj->values[3]->name}";

echo "This is the value of the var named $name: {${$name}}";

echo "This is the value of the var named by the return value of getName: {${getName}}";

echo "This is the value of the var named by the return value of \$object->getName: {${$object->getName}}";

// Won't work, outputs: This is the return value of getName: {getName} echo "This is the return value of getName: {getName}"; ?>

It is also possible to access class properties using variables within strings using this syntax. $bar}\n"; echo "{$foo->$baz[1]}\n"; ?>

The above example will output: I am bar. I am bar.

Note:

Functions, method calls, static class variables, and class constants inside {$} work since PHP 5. However, the value accessed will be interpreted as the name of a variable in the scope in which the string is defined. Using single curly braces ({}) will not work for accessing the return values of functions or methods or the values of class constants or static class variables.

 String access and modification by character

Characters within strings may be accessed and modified by specifying the zero-based offset of the desired character after the string using square array brackets, as in $str[42]. Think of a string as an array of characters for this purpose. The functions substr and substr_replace can be used when you want to extract or replace more than 1 character.

Note: Strings may also be accessed using braces, as in $str{42}, for the same purpose. Warning

Writing to an out of range offset pads the string with spaces. Non-integer types are converted to integer. Illegal offset type emits E_NOTICE. Negative offset emits E_NOTICE in write but reads empty string. Only the first character of an assigned string is used. Assigning empty string assigns NULL byte.

Example #9 Some string examples 

Note:

Accessing variables of other types (not including arrays or objects implementing the appropriate interfaces) using [] or {} silently returns NULL.

Useful functions and operators

Strings may be concatenated using the '.' (dot) operator. Note that the '+' (addition) operator will not work for this. See String operators for more information.

There are a number of useful functions for string manipulation.

See the string functions section for general functions, and the regular expression functions or the Perl-compatible regular expression functions for advanced find & replace functionality.

There are also functions for URL strings, and functions to encrypt/decrypt strings (mcrypt and mhash).

Finally, see also the character type functions. Converting to string

A value can be converted to a string using the (string) cast or the strval function. String conversion is automatically done in the scope of an expression where a string is needed. This happens when using the echo or print functions, or when a variable is compared to a string. The sections on Types and Type Juggling will make the following clearer. See also the settype function.

A boolean TRUE value is converted to the string "1". Boolean FALSE is converted to "" (the empty string). This allows conversion back and forth between boolean and string values.

An integer or float is converted to a string representing the number textually (including the exponent part for floats). Floating point numbers can be converted using exponential notation (4.1E+6).

Note:

The decimal point character is defined in the script's locale (category LC_NUMERIC). See the setlocale function.

Arrays are always converted to the string "Array"; because of this, echo and print can not by themselves show the contents of an array. To view a single element, use a construction such as echo $arr['foo']. See below for tips on viewing the entire contents.

Objects in PHP 4 are always converted to the string "Object". To print the values of object properties for debugging reasons, read the paragraphs below. To get an object's class name, use the get_class function. As of PHP 5, the __toString method is used when applicable.

Resources are always converted to strings with the structure "Resource id #1", where 1 is the unique number assigned to the resource by PHP at runtime. Do not rely upon this structure; it is subject to change. To get a resource's type, use the get_resource_type function.

NULL is always converted to an empty string.

As stated above, directly converting an array, object, or resource to a string does not provide any useful information about the value beyond its type. See the functions print_r and var_dump for more effective means of inspecting the contents of these types.

Most PHP values can also be converted to strings for permanent storage. This method is called serialization, and is performed by the serialize function. If the PHP engine was built with WDDX support, PHP values can also be serialized as well-formed XML text. String conversion to numbers

When a string is evaluated in a numeric context, the resulting value and type are determined as follows.

If the string does not contain any of the characters '.', 'e', or 'E' and the numeric value fits into integer type limits (as defined by PHP_INT_MAX), the string will be evaluated as an integer. In all other cases it will be evaluated as a float.

The value is given by the initial portion of the string. If the string starts with valid numeric data, this will be the value used. Otherwise, the value will be 0 (zero). Valid numeric data is an optional sign, followed by one or more digits (optionally containing a decimal point), followed by an optional exponent. The exponent is an 'e' or 'E' followed by one or more digits. 

For more information on this conversion, see the Unix manual page for strtod(3).

To test any of the examples in this section, cut and paste the examples and insert the following line to see what's going on: 

Do not expect to get the code of one character by converting it to integer, as is done in C. Use the ord and chr functions to convert between ASCII codes and characters. Details of the String Type

The string in PHP is implemented as an array of bytes and an integer indicating the length of the buffer. It has no information about how those bytes translate to characters, leaving that task to the programmer. There are no limitations on the values the string can be composed of; in particular, bytes with value 0 (“NUL bytes”) are allowed anywhere in the string (however, a few functions, said in this manual not to be “binary safe”, may hand off the strings to libraries that ignore data after a NUL byte.)

This nature of the string type explains why there is no separate “byte” type in PHP – strings take this role. Functions that return no textual data – for instance, arbitrary data read from a network socket – will still return strings.

Given that PHP does not dictate a specific encoding for strings, one might wonder how string literals are encoded. For instance, is the string "á" equivalent to "\xE1" (ISO-8859-1), "\xC3\xA1" (UTF-8, C form), "\x61\xCC\x81" (UTF-8, D form) or any other possible representation? The answer is that string will be encoded in whatever fashion it is encoded in the script file. Thus, if the script is written in ISO-8859-1, the string will be encoded in ISO-8859-1 and so on. However, this does not apply if Zend Multibyte is enabled; in that case, the script may be written in an arbitrary encoding (which is explicity declared or is detected) and then converted to a certain internal encoding, which is then the encoding that will be used for the string literals. Note that there are some constraints on the encoding of the script (or on the internal encoding, should Zend Multibyte be enabled) – this almost always means that this encoding should be a compatible superset of ASCII, such as UTF-8 or ISO-8859-1. Note, however, that state-dependent encodings where the same byte values can be used in initial and non-initial shift states may be problematic.

Of course, in order to useful, functions that operate on text may have to make some assumptions about how the string is encoded. Unfortunately, there is much variation on this matter throughout PHP’s functions: Some functions assume that the string is encoded in some (any) single-byte encoding, but they do not need to interpret those bytes as specific characters. This is case of, for instance, substr, strpos, strlen or strcmp. Another way to think of these functions is that operate on memory buffers, i.e., they work with bytes and byte offsets. Other functions are passed the encoding of the string, possibly they also assume a default if no such information is given. This is the case of htmlentities and the majority of the functions in the mbstring extension. Others use the current locale (see setlocale), but operate byte-by-byte. This is the case of strcasecmp, strtoupper and ucfirst. This means they can be used only with single-byte encodings, as long as the encoding is matched by the locale. For instance strtoupper("á") may return "Á" if the locale is correctly set and á is encoded with a single byte. If it is encoded in UTF-8, the correct result will not be returned and the resulting string may or may not be returned corrupted, depending on the current locale. Finally, they may just assume the string is using a specific encoding, usually UTF-8. This is the case of most functions in the intl extension and in the PCRE extension (in the last case, only when the u modifier is used). Although this is due to their special purpose, the function utf8_decode assumes a UTF-8 encoding and the function utf8_encode assumes an ISO-8859-1 encoding.

Ultimately, this means writing correct programs using Unicode depends on carefully avoiding functions that will not work and that most likely will corrupt the data and using instead the functions that do behave correctly, generally from the intl and mbstring extensions. However, using functions that can handle Unicode encodings is just the beginning. No matter the functions the language provides, it is essential to know the Unicode specification. For instance, a program that assumes there is only uppercase and lowercase is making a wrong assumption.