php check if values in form have been filled - php

Question

I have a form and php that submits the values, how do I make the php to check if the values are not empty?
Server code
$XX = mysqli_real_escape_string($link, $_REQUEST['XX']);
$YY = mysqli_real_escape_string($link, $_REQUEST['YY']);
if(empty($XX) || empty($YY))
{
echo "You need to fill in XX or YY";
}
Form markup:
<form method="POST" action="">
<label for="XX">XX</label><br>
<label for="YY">YY</label><br>
<input type="text" name="XX" id="XX"><br>
<input type="text" name="YY" id="YY"><br>
<input class="button" type="submit" value="submit"><br>
</form>

Answer 1

Assuming you are trying to check that at least one of these inputs has been set as your echoed message suggests then you need to use an and && and not an or || like this
if(empty($XX) && empty($YY))
{
echo "You need to fill in XX or YY";
}

Answer 2

PHP has three useful functions to test the value of a variable, you need to understand how these functions work in order to use them properly, below is a short description for each, hope it helps
isset()
Determines if a variable is set and is NOT NULL
So if the value assigned is "" or 0 or “0” or false the return will be true, if is NULL it will return false.
$var = '';
if(isset($var)) {
echo 'The variable $var is set.';
}
unset($var);
if(!sset($var)) {
echo 'The variable $var is not set';
}
Empty()
Determines if a variable is empty
So if the value is "" or 0 or 0.0 or "0" or NULL or False or [] it will return true
$var = '';
if(empty($var)) {
echo 'The variable $var is empty or not set';
}
is_null()
It returns true only if a variable is NULL.
$var = NULL;
if(is_null($var)) {
echo 'The variable $var is NULL';
}
if(is_null($foo)) {
echo 'The variable $foo is inexistent so the value is NULL and will evaluate to true';
}