chairs.php

<?php

$a = range(1,100);
while(count($a) > 1) {
    $key = key($a);
    unset($a[$key]);
    if ( false === current($a) ) {
        reset($a);
    }
    if ( false === next($a) ) {
        reset($a);
    }
}
echo reset($a), PHP_EOL;