<?php
$error = [];
$requireds = ['field1','field2','field3'];
if (ISSET($_POST['field1'],$_POST['field2'],$_POST['field3'])) {
    // do something
    foreach ($requireds as $required) {
        if (array_key_exists($required, $_POST)) {
            switch ($required) {
                case 'field1':
                    if ($_POST[$required] != 'your condition') {
                        $error[] = 'Validate field1 failed';
                    } else {
                        $field1 = $_POST[$field1];
                    }
                break;
                case 'field2':
                    if ($_POST[$required] != 'your condition') {
                        $error[] = 'Validate field2 failed';
                    } else {
                        $field2 = $_POST[$field2];
                    }
                break;
                default:
                break;
            }
        } else {
            $error[] = 'Missing required field '. $required;
        }
    }



    if (count($error) > 0) {
        // there are some errors;
        // do something to handle your errors;
    } else {
        // validation pass;
        // do what you want to do with $field1 and $field2;
    }
}
?>