Again, from Codility. Need to transform the string regarding the rues as long as there are still matching rules.

string-transformation.php

<?php

function solution($S)
{
    $rules = [
        'AB' => 'AA',
        'BA' => 'AA',
        'CB' => 'CC',
        'BC' => 'CC',
        'AA' => 'A',
        'CC' => 'C',
    ];

    $count = 1;

    do {
        $matchedRules = array_reduce(array_keys($rules), function($matchedRules, $pattern) use ($S, $rules) {
            if (false !== strpos($S, $pattern)) {
                $matchedRules[] = [$pattern => $rules[$pattern]];
            }
            return $matchedRules;
        }, []);

        if ($matchedRules) {
            $ruleIdx = array_rand($matchedRules);

            foreach($matchedRules[$ruleIdx] as $search => $replace) {
                $S = str_replace($search, $replace, $S, $count);
            }
        }
    } while ($matchedRules);

    return $S;
}

echo solution('ABBCC');