Strings

Syntax

Ein String-Literal kann auf vier unterschiedliche Arten spezifiziert werden:

• einfache Anführungszeichen
• doppelte Anführungszeichen
• Heredoc Syntax
• Nowdoc Syntax (seit PHP 5.3.0)

My name is "MyName". I am printing some Foo.
Now I am printing some Bar2.
This should print a capital 'A': \x41

Example of string spanning multiple lines
using nowdoc syntax. Backslashes are always treated literally,
e.g. \\ and \'.

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

He drank some apple juice.
He drank some juice made of .
He drank some juice made of apples.

He drank some apple juice.
He drank some orange juice.
He drank some purple juice.
John Smith drank some apple juice.
John Smith then said hello to Jane Smith.
John Smith's wife greeted Robert Paulsen.
Robert Paulsen greeted the two .

The character at index -2 is n.
Changing the character at index -3 to o gives strong.

I am bar.
I am bar.

string(1) "b"
bool(true)
string(1) "b"
bool(true)
string(1) "a"
bool(true)
string(1) "b"
bool(true)

string(1) "b"
bool(true)

Warning: Illegal string offset '1.0' in /tmp/t.php on line 7
string(1) "b"
bool(false)

Warning: Illegal string offset 'x' in /tmp/t.php on line 9
string(1) "a"
bool(false)
string(1) "b"
bool(false)

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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 Perl-compatible regular expression functions for advanced find & replace functionality.

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

Finally, see also the character type functions.

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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).

Hinweis:

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.

In order to convert objects to string magic method __toString must be used.

Resources are always converted to strings with the structure "Resource id #1", where 1 is the resource number assigned to the resource by PHP at runtime. While the exact structure of this string should not be relied on and is subject to change, it will always be unique for a given resource within the lifetime of a script being executed (ie a Web request or CLI process) and won't be reused. 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.

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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.

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Details des String-Typs

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 be 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.