Extract file marker to get the file type

$png = [
    'format' => 'H32',
    'marker' => '89504e470d0a1a0a0000000d49484452'
];

$bytes_to_read = strlen( $png['marker'] ) / 2;

$fp = fopen('your-filename.png', 'r');
$data = fread($fp, $bytes_to_read);
fclose($fp);

$x = unpack( $png['format'], $data);

$is_png = ( $x[1] == strtolower( $png['marker'] ) ) ? true : false;

var_dump( $is_png );