PHP – array_diff does not work as it supposed to

June 28, 2022

Just found a strange behavior of PHP with the array_diff function.

I have the following arrays:

    $d1 = [
        'HomePhoneNumber' => '555-222-2222',
        'MobilePhoneNumber' => NULL,
        'ContactID' => NULL,
        'ApplicantID' => '2'
    ];
    
    
    $d2 = [
        'HomePhoneNumber' => '555-222-2222',
        'MobilePhoneNumber' => '555-222-3333',
        'ContactID' => '1',
        'ApplicantID' => '2'
    ];

I need to find the values from $d2 which are not present into $d1:

array_diff($d2, $d1)

The function returns me:

array (
  'MobilePhoneNumber' => '555-222-3333',
  'ContactID' => '1',
)

This is correct!

But, if I compare the same arrays with a different value of 'ContactID' key, the array_diff() function returns me a different result ('ContactID' is no longer returned):

    $d1 = [
        'HomePhoneNumber' => '555-222-2222',
        'MobilePhoneNumber' => NULL,
        'ContactID' => NULL,
        'ApplicantID' => '2'
    ];
    
    
    $d2 = [
        'HomePhoneNumber' => '555-222-2222',
        'MobilePhoneNumber' => '555-222-3333',
        'ContactID' => '2',
        'ApplicantID' => '2'
    ];

// Result:
array (
  'MobilePhoneNumber' => '555-222-3333',
)

Do you know why? I don’t understand.
Thanks for you help!