SecretSanta.php

<?php
$userNames = str_split('QWERTYUIJPABCDE');
$forbidden = [
	'Q' => 'W',
	'P' => 'A',
	'U' => 'I',
	'D' => 'E',
];

$generateSet = function() use ($userNames){
	$randUsers = $userNames;
	shuffle($randUsers);
	return array_combine($randUsers, $userNames);
};

$isValidSet = function($set) use ($forbidden){
	foreach($set as $k=>$v){
		if(($k == $v) || (isset($forbidden[$k]) && $forbidden[$k] == $v)){
			return false;
		}
	}
	return true;
};

do{
	$set = $generateSet();
}while(!$isValidSet($set));

foreach($set as $k => $v){
	echo $k . " dāvina " . $v . "<br>";
}