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Strings
A string is series of characters. Before PHP 6, a character is
the same as a byte. That is, there are exactly 256 different characters
possible. This also implies that PHP has no native support of Unicode. See
utf8_encode() and utf8_decode() for some
basic Unicode functionality.
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 computer on which PHP is running.
Syntax
A string literal can be specified in four different ways:
Single quoted
The simplest way to specify a string is to enclose it in single
quotes (the character ').
To specify a literal single quote, escape it with a backslash
(\). To specify a literal backslash before a single
quote, or at the end of the string, double it
(\\). Note that attempting to escape any other character
will print the backslash too.
Note:
Unlike the two other syntaxes,
variables and escape sequences
for special characters will not be expanded when they
occur in single quoted strings.
Double quoted
If the string is enclosed in double-quotes ("), PHP will
interpret more escape sequences for special characters:
Escaped characters
| Sequence |
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
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As in single quoted strings, escaping any other character will
result in the backslash being printed too. Before PHP 5.1.1, the backslash
in \{$var} was not been printed.
The most important feature of double-quoted strings is the fact
that variable names will be expanded. See
string parsing for
details.
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 members. Use
nowdocs instead.
Example #1 Invalid example
<?php
class foo {
public $bar = <<<EOT
bar
EOT;
}
?>
Heredoc text behaves just like a double-quoted string, without
the double quotes. This means that quotes in a heredoc do 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 #2 Heredoc string quoting example
<?php
$str = <<<EOD
Example of string
spanning multiple lines
using heredoc syntax.
EOD;
/* More complex example, with variables. */
class foo
{
var $foo;
var $bar;
function foo()
{
$this->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 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 Note:
Heredoc support was added in PHP 4.
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 <<<
seqeuence 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 #3 Nowdoc string quoting example
<?php
$str = <<<'EOD'
Example of string
spanning multiple lines
using nowdoc syntax.
EOD;
/* More complex example, with variables. */
class foo
{
public $foo;
public $bar;
function foo()
{
$this->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 members or constants:
Example #4 Static data example
<?php
class foo {
public $bar = <<<'EOT'
bar
EOT;
}
?>
Note:
Nowdoc support was added in PHP 5.3.0.
Variable parsing
When a string is specified in double quotes or with heredoc,
variables are parsed within it.
There are two types of syntax: a
simple one and a
complex one.
The simple syntax is the most common and convenient. It provides a way to
embed a variable, an array value, or an object
property in a string with a minimum of effort.
The complex syntax was introduced in PHP 4, and can be recognised by the
curly braces surrounding the expression.
Simple syntax
If a dollar sign ($) 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.
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.
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.
In fact, any value in the namespace can be included in a
string with this syntax. Simply write the expression the same
way as it would appeared 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
{$. Some examples to make it clear:
Note:
Functions and method calls inside {$} work since PHP 5.
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.
Note:
Strings may also be accessed using braces, as in
$str{42}, for the same purpose. However, this syntax is
deprecated as of PHP 6. Use square brackets instead.
Example #5 Some string examples
<?php
// Get the first character of a string
$str = 'This is a test.';
$first = $str[0];
// Get the third character of a string
$third = $str[2];
// Get the last character of a string.
$str = 'This is still a test.';
$last = $str[strlen($str)-1];
// Modify the last character of a string
$str = 'Look at the sea';
$str[strlen($str)-1] = 'e';
?>
Note:
Accessing variables of other types using [] or
{} silently returns NULL.
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 members 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.
The string will be evaluated as a float if it
contains any of the characters '.', 'e', or 'E'. Otherwise, it will be
evaluated as an integer.
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.
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