In case you want to use an open polygon but are stuck with a PHP version prior to 7.2, a solution may be to 'backplot' your array to its original start. Say you have an array of pixels (below seperated by commas)

<?php
$arr = array();
for ($i = 0; $i < count($pixels); $i++) {
    $pixel = explode(',', $pixels[$i]);
    if (($pixel[0] > 0) && ($pixel[1] > 0)) {
        $arr[] = $pixel[0];
        $arr[] = $pixel[1];
    }
}
imagepolygon($im, $arr, (count($arr) / 2), $otcolor);
?>

you can replace this by something like

<?php
$arr = array();
for ($i = 0; $i < count($pixels); $i++) {
    $pixel = explode(',', $pixels[$i]);
    $arr[] = $pixel[0];
    $arr[] = $pixel[1];
}
// imageopenpolygon($im, $arr, (count($arr) / 2), $otcolor) is not possible, so...
for ($i = (count($pixels)-1); $i >= 0; $i--) {
    $pixel = explode(',', $pixels[$i]);
    $arr[] = $pixel[0];
    $arr[] = $pixel[1];
}
imagepolygon($im, $arr, (count($arr) / 2), $otcolor);
?>