Compare 2 strings and return the common element

Is something like this built in to PHP?

[penders]

So I want to return the common string between two strings, starting from the beginning - is there already a PHP function that will do something like this? It kinda feels as if there should be but I can't find it (before I go write one).

ie.

$str1 = "HelloWorld" 
$str2 = "HelloBob"

Comparing $str1 and $str2 would return:

"Hello"

[penders]

Well, I couldn't find a PHP function to do the job, so I wrote one...

function strCommon($str1,$str2) { 
 $common = false; 
 $maxlen = min(strlen($str1),strlen($str2)); 
 for ($i=0; $i<$maxlen; $i++) { 
  if ($str1[$i] == $str2[$i]) { 
   $common .= $str1[$i]; 
  } else { 
   break; 
  } 
 } 
 return $common; 
}

Returns (bool)false if no match.