(PHP 4 >= 4.3.0, PHP 5)
mb_substr_count — Count the number of substring occurrences
$haystack
   , string $needle
   [, string $encoding = mb_internal_encoding()
  ] )
   Counts the number of times the
   needle substring occurs in the
   haystack string.
  
   The number of times the
   needle substring occurs in the
   haystack string.
  
Example #1 mb_substr_count() example
<?php
echo mb_substr_count("This is a test", "is"); // prints out 2
?>