Reverse string without strrev

$string = 'abc';

$reverted = implode(array_reverse(str_split($string)));

You could use the XOR swap trick.

function rev($str) {
    $len = strlen($str);

    for($i = 0; $i < floor($len / 2); ++$i) {
        $str[$i] = $str[$i] ^ $str[$len - $i - 1];
        $str[$len - $i - 1] = $str[$i] ^ $str[$len - $i - 1];
        $str[$i] = $str[$i] ^ $str[$len - $i - 1];
    }

    return $str;
}

print rev("example");

You can use the fact that in PHP a string can be thought of as an array of characters.

Then basically what you want to do is to replace each character $i on the left side of the middle of the string with the character $j on the right side of the middle with the same distance.

For example, in a string of seven characters the middle character is on position 3. The character on position 0 (distance 3) needs to be swapped with the character on position 6 (3 + 3), the character on position 1 (distance 2) needs to be swapped with the character on position 5 (3 + 2), etc.

This algorithm can be implemented as follows:

$s = 'abcdefg';

$length = strlen($s); 
for ($i = 0, $j = $length-1; $i < ($length / 2); $i++, $j--) {
    $t = $s[$i];
    $s[$i] = $s[$j];
    $s[$j] = $t;
}

var_dump($s);

That's an interesting one. Here's something I just came up with:

$s = 'abcdefghijklm';
for($i=strlen($s)-1, $j=0; $j<$i; $i--, $j++) {
    list($s[$j], $s[$i]) = array($s[$i], $s[$j]);
}
echo $s;

list() can be used to assign a list of variables in one operation. So what I am doing is simply swapping characters (starting with first and last, then second-first and second-last and so on, till it reaches the middle of the string)

Output is mlkjihgfedcba. Not using any other variables than $s and the counters, so I hope that fits your criteria.