I want to stress that in the user function, you do need to return either a 1 or a -1 properly; you cannot simply return 0 if the results are equal and 1 if they are not.  

The following code is incorrect:

<?php
function myfunction($v1,$v2) 
{
if ($v1===$v2)
    {
    return 0;
    }
return 1;
}

$a1=array(1, 2, 4);
$a2=array(1, 3, 4);
print_r(array_uintersect($a1,$a2,"myfunction"));
?>

This code is correct:

<?php
function myfunction($v1,$v2) 
{
if ($v1===$v2)
    {
    return 0;
    }
if ($v1 > $v2) return 1;
return -1;
}
$a1=array(1, 2, 4);
$a2=array(1, 3, 4);
print_r(array_uintersect($a1,$a2,"myfunction"));
?>

If you're rolling your own comparison function, keep in mind that the spaceship operator (i.e. <=>) can be your best friend. It's been around since PHP7. https://www.php.net/manual/en/language.operators.comparison.php

So, for instance, instead of a clunky function like:

<?php
function myFunction($v1, $v2) {
   if ($v1 === $v2) {
      return 0;
   }
   if ($v1 > $v2) return 1;
   return -1;
}
?>

You can simplify it to:

<?php
function myFunction($v1, $v2) {
   return $v1 <=> $v2;
}
?>

When trying to do a case insensitive comparison between arrays of words, the strcasecmp function works very nicely with this one like so: 

$arr1 = array('blue', 'green', 'red');

$arr2 = array('BLUE', 'Purple', 'Red');

$loose_matches = array_uintersect($arr1, $arr2, 'strcasecmp');

print_r($loose_matches) // array('blue', 'red');

As for the other "compare function" callbacks, the return value from the callback function doesn't need to be -1, 0, or 1.

cmp($a,$b) just needs to be <0, =0, or >0 depending on whether $a<$b, $a=$b, or $a>$b.