Bullshit

index.php

<?php

$string = "A,B,C,D,'1,2,3,4,5,6',F,G";

$boom = explode("'", $string)[1];

$new_string = str_replace($boom, "", $string);

foreach (explode(",", $new_string) as $letter) {
    if ($letter != "''") {
        $array[] = $letter;    
    } else {
        $array[] = $boom;
    }
}

echo '<pre>'.print_r($array, true).'</pre>';