One of available algorithm steps:

1. Compare neighbour symbols. Use loop for it.

2. If symbols are equal, replace 2 symbols in String and change search index to 0

    for($i = 1; $i < strlen($s); $i++){
        if( $s[$i] === $s[$i-1] ){
            $s = substr_replace($s, '', $i-1, 2);
            $i = 0;
        }
    }
    return $s;