<form>
    N:<input type="text" name="num1">
    M:<input type="text" name="num2">
</form>
<?php
$num1 = 0;
$num2 = 0;
if(isset($_GET["num1"], $_GET["num2"])) {
    $num1 = intval($_GET["num1"]);
    $num2 = intval($_GET["num2"]);
}
for($list=1;$list<=$num1;$list++) {
    echo "<ul><li>List $list<ul>";
    for($el=1;$el<=$num2;$el++) {
        echo "<li>Element $list.$el</li>";
    }
    echo "</ul></li></ul>";
}