validate.php

<?php

$expected = ["name", "email", "comments"];
$required = ["name", "comments"];

foreach ($_POST as $key => $value) {
    if (!is_array($value)) {
        $value = trim($value);
    }

    if (!in_array($key, $expected)) {
        // ignore the value, it's not in $expected
        continue;       
    }
    
    if (!in_array($key, $required)) {
        // optional value, which we'll default to empty string
        $$key = $value ?? "";
        continue;
    }
    
    if (empty($value)) {
        // required value is missing
        $missing[] = $key;
        $$key = "";
        continue;
    }

    $$key = $value;
}