// Created on savesnippets.com · https://savesnippets.com/KvXTwRvz3z4Iph
<?php
function similarity(string $a, string $b): float {
    if ($a === $b)             return 1.0;
    if ($a === '' || $b === '') return 0.0;
    $maxLen = max(strlen($a), strlen($b));
    return 1.0 - (levenshtein($a, $b) / $maxLen);
}

printf("%.2f\n", similarity('kitten', 'sitting'));   // 0.57
printf("%.2f\n", similarity('php', 'phpx'));         // 0.75
printf("%.2f\n", similarity('hello', 'hello'));      // 1.00