What Restrictions Apply to PHP Array Keys?

Are All Characters Permitted as PHP Array Keys?

PHP arrays allow for a wide range of key types, including integers and strings. However, limitations exist as to which characters may be used in string keys.

Key Type Restrictions

As per the PHP manual, the following restrictions apply to array keys:

• Strings containing valid integers are automatically cast as integers (e.g., "8" becomes 8).
• Floats are also cast to integers, truncating any decimal portion (e.g., 8.7 becomes 8).
• Booleans are converted to integers (true becomes 1, false becomes 0).
• Null values are cast as an empty string ("").
• Arrays and objects cannot serve as keys, triggering a warning.

String Key Restrictions

Regarding string keys, the manual notes that PHP stores characters as bytes. Therefore, each character in a key must be within the supported 256-character set. This means that PHP does not natively support Unicode.

Allowed Characters

In essence, any string can be used as an array key in PHP. This includes any sequence of characters, even binary data, as long as it conforms to the 256-character limitation.

Example

The following code demonstrates some unconventional but valid uses of array keys:

<code class="php">$w = array(chr(0) => 'null byte?', chr(rand(0, 255)) => 'random byte?');
var_dump($w);</code>

This code initializes an array with a key containing a null byte (chr(0)) and another key containing a random byte (chr(rand(0, 255))).

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