PHPで文字列をリバースする

reverse.php

function reverse1($str1) {
    $count = strlen($str1);
    for ($i = $count - 1; $i >= 0; $i--) {
        echo $str1[$i];
    }
}

$str1 = "abcdef";
reverse1($str1);

reverse2.php

function reverse2($str2) {
    $count = strlen($str2);
    $num   = $count - 1;
    for ($i = 0; $i < $count / 2; $i++) {
        $last = $str2[$i];
        $str2[$i] = $str2[$num];
        $str2[$num] = $last;
        $num--;
    }
    echo $str2;
}

$str2 = "abcdef";
reverse2($str2);

reverse3.php

function reverse3($str3) {
    $count = strlen($str3);
    for($i = 0; $i < --$count; $i++) {
        $last = $str3[$i];
        $str3[$i] = $str3[$count];
        $str3[$count] = $last;
    }
    echo $str3;
}

$str3 = "abcdef";
reverse3($str3);