<?php 
/** 
 *  
 * @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 "<br>"; 
var_dump(password_strength("Super Monkey Ball")); 
echo "<br>"; 
var_dump(password_strength("Tr0ub4dor&3")); 
echo "<br>"; 
var_dump(password_strength("abc123")); 
echo "<br>"; 
var_dump(password_strength("sweet"));