[PHP-CLI] Ulam spiral

<?php
function is_prime($n) {
    $s = 0;
    for ($i = 1; $i <= floor(sqrt($n))+1; $i++) {
        $s += floor($n/$i)-floor(($n-1)/$i);
    }
    if ($s == 1) { return true; }
    else { return false; }
}
$height = 768;
$width  = 1366;
$top    = $height/4;
$left   = $width/4;
$move   = 0;
$trace  = 1;
$traceB = 1;
$count  = 0;
$next   = 0;
//0 : move right
//1 : move top
//2 : move left
//3 : move bottom
$str = "";
for ($i = 2; $i <= 100000; $i++) {
    if ($count == $trace && $next == 0) {
        $count = 0; $next = 1; $move++; $move = $move % 4;
    } elseif ($count == $trace && $next == 1) {
        $count = 0; $next = 0; $trace++; $move++; $move = $move % 4;
    }
    if ($count < $trace) {
        $count++;
    }
    if ($move == 0) { $left++; }
    elseif ($move == 1) { $top++; }
    elseif ($move == 2) { $left--; }
    elseif ($move == 3) { $top--; }
    if (is_prime($i)) { $str .= "<div style='position: absolute; left: ".(2*$left)."px; bottom: ".(2*$top)."px'>.".
        "</div>"; }
    $str.="</div>";
    if ($i % 1000 == 0) { print "$i\n"; }
}
file_put_contents(".html",$str);
?>