PHP Cookbook: Extracting Substrings

Problem
You want to extract part of a string, starting at a particular place in the string. For example, you want the first eight characters of a username entered into a form.

Solution
Use substr( ) to select your substring, as in Example 1-9. Example 1-9. Extracting a substring with substr( )

<?php
$substring = substr($string,$start,$length);
$username = substr($_GET['username'],0,8);
?>

Discussion
If $start and $length are positive, substr( ) returns $length characters in the string, starting at $start. The first character in the string is at position 0. Example 1-10 has positive $start and $length.

print substr('watch out for that tree',6,5);

Example 1-10 prints:
out f

If you leave out $length, substr( ) returns the string from $start to the end of the original string, as shown in Example 1-11.

Example 1-11. Using substr( ) with positive start and no length

print substr('watch out for that tree',17);

Example 1-11 prints:
t tree

If $start is bigger than the length of the string, substr( ) returns false..
If $start plus $length goes past the end of the string, substr( ) returns all of the string from $start forward, as shown in Example 1-12.

Example 1-12. Using substr( ) with length past the end of the string

print substr('watch out for that tree',20,5);

Example 1-12 prints:
ree

If $start is negative, substr( ) counts back from the end of the string to determine where your substring starts, as shown in Example 1-13.

Example 1-13. Using substr( ) with negative start

print substr('watch out for that tree',-6);
print substr('watch out for that tree',-17,5);

Example 1-13 prints:
t tree
out f

With a negative $start value that goes past the beginning of the string (for example, if $start is −27 with a 20-character string), substr( ) behaves as if $start is 0.

If $length is negative, substr( ) counts back from the end of the string to determine where your substring ends, as shown in Example 1-14.

Example 1-14. Using substr( ) with negative length

print substr('watch out for that tree',15,-2);
print substr('watch out for that tree',-4,-1);

Example 1-14 prints:

hat tr
tre

See Also
Documentation on substr( ) at http://www.php.net/substr.

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