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<?php

function reverse(string $str) {
    $first = 0;
    $last = strlen($str) - 1;
    while ($first < $last) {
        $tmpFirst = $str[$first];
        $str[$first] = $str[$last];
        $str[$last] = $tmpFirst;
        $first++;
        $last--;
    }
    
    return $str;
}

echo reverse('abc'), PHP_EOL;