php calculate password strength

The following php code is used to test the strength of a given password, the maximum strength is 100

/**
 *
 * @param String $string
 * @return float
 *
 * Returns a float between 0 and 100. The closer the number is to 100 the
 * the stronger password is; further from 100 the weaker the password is.
*/
function password_strength($string){
 $h = 0;
 $size = strlen($string);
 foreach(count_chars($string, 1) as $v){
 $p = $v / $size;
 $h -= $p * log($p) / log (2);
 }
 $strength = ($h / 4) * 100;
 if($strength > 100){
 $strength = 100;
 }
 return $strength;
}

var_dump(password_strength( "Correct Horse Battery Staple"));
echo "";
var_dump(password_strength("Super Monkey Ball"));
echo "";
var_dump(password_strength("Tr0ub4dor&3")) ;
echo "";
var_dump(password_strength("abc123"));
echo "";
var_dump(password_strength("sweet"));

php