PHP create the optgroups based on the value of an array

Based on your given example, I divided the option group based on your keywords, first i used the strpos to get the position of these: lg, md, sm and xs, then i created the name of the option group based on your example. After I created the new array and group each of those values on their designated groups, I started iterating the option group and select. Check this:

$YPE_grid = array( '1 Col/1 row(lg)' => 12.1, '2 Col/1 row(lg)' => 6.2, '3 Col/1 row(lg)' => 4.3, '4 Col/1 row(lg)' => 3.4, '6 Col/1 row(lg)' => 2.5, '12 Col/1 row(lg)' => 1.6 , '1 Col/1 row(md)' => 12.7, '2 Col/1 row(md)' => 6.8, '3 Col/1 row(md)' => 4.9, '4 Col/1 row(md)' => 3.10, '6 Col/1 row(md)' => 2.11, '12 Col/1 row(md)' => 1.12, '1 Col/1 row(sm)' => 12.13, '2 Col/1 row(sm)' => 6.14, '3 Col/1 row(sm)' => 4.15, '4 Col/1 row(sm)' => 3.16, '6 Col/1 row(sm)' => 2.17, '12 Col/1 row(sm)' => 1.18, '1 Col/1 row(xs)' => 12.19, '2 Col/1 row(xs)' => 6.21, '3 Col/1 row(xs)' => 4.22, '4 Col/1 row(xs)' => 3.23, '6 Col/1 row(xs)' => 2.24, '12 Col/1 row(xs)' => 1.25 );

    $newArray = array();
    foreach($YPE_grid as $key => $value) {
        if(strpos($key,"lg") > 0) {
            $newArray['"Large Devices"'][$key] = $value;
        } else if(strpos($key,"md") > 0) {
            $newArray['"Medium devices"'][$key] = $value;
        } else if(strpos($key,"sm") > 0) {
            $newArray['"Small devices"'][$key] = $value;
        } else if(strpos($key,"xs") > 0) {
            $newArray['"Extra small devices"'][$key] = $value;
        }
     }

$select = "";
$select .= "<select>";
foreach($newArray as $key => $value) {
    $select .= "<optgroup label=" . $key . ">";
        if(is_array($value)) {
            foreach($value as $key2 => $value2) {
                $select .= "<option value='" . $key2 . "'>" . $key2 . "(" . $value2 . ")</option>";
            }
        }
    $select .= "</optgroup>";
}

$select .= "</select>";
echo $select;