Unexpected behavior of preg_replace_callback

I think the preg_replace_callback function of php has a runtime problem :

$a = [
        'test.db',
        'WhatsApp Image 2021-10-21 at 18.52.14.jpeg',
        'WhatsApp Image 2021-10-21 at 18.52.15.jpeg',
        'WhatsApp Image 2021-10-21 at 18.52.16.jpeg',
        'WhatsApp Image 2021-10-21 at 18.52.18 (1).jpeg',
        
       ];
    
     $callback = function ($matches) {
            return $matches[0];
        };
        
    $c = preg_replace_callback('/.*\.[jp][pn]e?g/i', $callback, $a );
        
    print_r($c);

return :

Array
(
    [0] => test.db
    [1] => WhatsApp Image 2021-10-21 at 18.52.14.jpeg
    [2] => WhatsApp Image 2021-10-21 at 18.52.15.jpeg
    [3] => WhatsApp Image 2021-10-21 at 18.52.16.jpeg
    [4] => WhatsApp Image 2021-10-21 at 18.52.18 (1).jpeg
)

Why is test.db still present when even when I parse the return $matches[0] it never fits inside ?

>Solution :

preg_replace_callback runs a replacement on each item in the array, but if there’s nothing to replace, it still returns the element unchanged.

preg_filter is what you need.

preg_filter() is identical to preg_replace() except it only returns the (possibly transformed) subjects where there was a match.

Side note: /.*\.[jp][pn]e?g/i will match foo.jpg.exe. You probably want /.*\.[jp][pn]e?g$/i instead.