Remove elements from a flat array if found in a subarray of a multidimensional array

I have 2 arrays:

$network_keys = ['7 ADELAIDE', '7TWO ADELAIDE'];

$dblist = [
    [
        'PrpMedia' => [
            'id' => 3,
            'network_key' => '7 ADELAIDE',
            'media_id' => 43
        ],
        'Media' => [
            'id' => 43,
            'media_type_id' => 5,
            'media_location_id' => 4,
            'media_name' => 'Channel 7 ADELAIDE',
            'deleted' => false
        ]
    ],
    [
        'PrpMedia' => [
            'id' => 4,
            'network_key' => 'VIC 1HD BENDIGO',
            'media_id' => 42
        ],
        'Media' => [
            'id' => 42,
            'media_type_id' => 5,
            'media_location_id' => 4,
            'media_name' => '7MATE',
            'deleted' => false
        ]
    ]
];

I want to find which values from $network_keys do not exist in $dbList.

To be clear, we are looking in the ['PrpMedia']['network_key'] subarray of each $dblist row.

Expected array:

$difference = ['7TWO ADELAIDE'];

You could do it with a combination of array_map and array_diff:

$diff = array_diff($network_keys, array_map(function($v) {
    return $v['PrpMedia']['network_key'];
}, $dblist));

In the example you give, you will get the value of $network_keys, because none of them occur in the searched array.

The code first extracts all the ['PrpMedia']['network_key'] values from $network_keys into an array that will have the same flat structure as $dblist. Then it simply takes the difference, only keeping the elements of $dblist that do not occur in that intermediate array.

Be aware that this code assumes the searched key ['PrpMedia']['network_key'] exists in all of the elements of $network_keys. If this cannot be assumed to be always the case, then add a check, like this:

$diff = array_diff($network_keys, array_map(function($v) {
    return isset($v['PrpMedia']['network_key']) 
        ? $v['PrpMedia']['network_key']
        : null;
}, $dblist));

This will also work correctly if the ['PrpMedia'] key is not present for one of the elements.