<?php

function t1($hay, $needle, $replacement) {
    return str_replace($needle, $replacement, $hay);
}
function t2($hay, $needle, $replacement) {
    return strtr($hay, [$needle => $replacement]);
}

function bench($fn, $n=1000000) {
    $start = microtime(true);
    $s = str_repeat('aaaahibbbb', 1000);
    for ($i = 0; $i < $n; ++$i) {
        $res = $fn($s, 'hi', 'hello');
    }

    echo (microtime(true) - $start), PHP_EOL;
    return $res;
}

bench('t1');
bench('t2');