(PHP 4, PHP 5, PHP 7)
preg_split — Split string by a regular expression
$pattern
   , string $subject
   [, int $limit = -1
   [, int $flags = 0
  ]] )Split the given string by a regular expression.
patternThe pattern to search for, as a string.
subjectThe input string.
limit
       If specified, then only substrings up to limit
       are returned with the rest of the string being placed in the last
       substring.  A limit of -1, 0 or NULL means "no limit"
       and, as is standard across PHP, you can use NULL to skip to the 
       flags parameter.
      
flags
       flags can be any combination of the following
       flags (combined with the | bitwise operator):
       
PREG_SPLIT_NO_EMPTYPREG_SPLIT_DELIM_CAPTUREPREG_SPLIT_OFFSET_CAPTURE
           If this flag is set, for every occurring match the appendant string
           offset will also be returned. Note that this changes the return
           value in an array where every element is an array consisting of the
           matched string at offset 0 and its string offset
           into subject at offset 1.
          
   Returns an array containing substrings of subject
   split along boundaries matched by pattern.
  
Example #1 preg_split() example : Get the parts of a search string
<?php
// split the phrase by any number of commas or space characters,
// which include " ", \r, \t, \n and \f
$keywords = preg_split("/[\s,]+/", "hypertext language, programming");
print_r($keywords);
?>
The above example will output:
Array
(
    [0] => hypertext
    [1] => language
    [2] => programming
)
Example #2 Splitting a string into component characters
<?php
$str = 'string';
$chars = preg_split('//', $str, -1, PREG_SPLIT_NO_EMPTY);
print_r($chars);
?>
The above example will output:
Array
(
    [0] => s
    [1] => t
    [2] => r
    [3] => i
    [4] => n
    [5] => g
)
Example #3 Splitting a string into matches and their offsets
<?php
$str = 'hypertext language programming';
$chars = preg_split('/ /', $str, -1, PREG_SPLIT_OFFSET_CAPTURE);
print_r($chars);
?>
The above example will output:
Array
(
    [0] => Array
        (
            [0] => hypertext
            [1] => 0
        )
    [1] => Array
        (
            [0] => language
            [1] => 10
        )
    [2] => Array
        (
            [0] => programming
            [1] => 19
        )
)
If you don't need the power of regular expressions, you can choose faster (albeit simpler) alternatives like explode() or str_split().
If matching fails, an array with a single element containing the input string will be returned.