str_common.php

<?php

/**
 * Return the common parts from 2 strings.
 *
 * @param string $str1
 * @param string $str2
 * @param bool $from_end
 * return string
 */
function str_common(string $str1, string $str2, $from_end = false) : string
{
    if ($from_end) {
        $str1 = strrev($str1);
        $str2 = strrev($str2);
    }
    
    for ($i=0; $i < strlen($str1); $i++) {
        if ($str1[$i] !== $str2[$i]) {
            break;
        }
    }
    
    $return = substr($str1, 0, $i);
    
    return $from_end ? strrev($return) : $return;
}

// Example usage:
$ver1 = 'The quick brown fox jumped over the lazy dog.';
$ver2 = 'The quick brown fox leapt over the lazy dog.';

echo str_common($ver1, $ver2);

// -> 'The quick brown fox '

echo str_common($ver1, $ver2, true);

// -> ' jumped over the lazy dog.'