In my images folder have file

1_cover.???
2_cover.???
3_cover.???
4_cover.???
5_cover.???

I wanna get file extension 4_cover.??? How to write PHP code

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UPDATE

Thanks for all help me, I can use this code

$images = glob("./images/4_cover.*");
print_r($images);

Is that what you are looking for ?

$info = new SplFileInfo('photo.jpg');
$path = $info->getExtension();
var_dump($path);

PHP Documentation

If you want to look in a directory for files, this might not be the best suited way to do your method but since you don't know what the file-type is, you can do something like this: (all code should be in order from top-bottom)

The directory housing all of your files

$directory = "public/images/headers/*";

The files gathered from the glob function, use print_r($files) to see all of the files gathered for debugging if there's an error going on

$files = glob( $directory );

The file you said you were looking for, if this is from a database you'll replace this data with data from the database

$filename_to_lookfor = '4_cover.';

If statements to check the file types and see if they're existant

$file_types_to_check_for = ['gif', 'jpg', 'png'];

foreach ($file_types_to_check_for as $filetype)
  if (in_array( $filename_to_lookfor.$filetype, $files)
  echo "This is a {$filetype} file!";

After reading more into glob - I'm not too experienced with it.

You can simply write this line:

if (count($files = glob( 'public/images/4_cover.*' )) != 0) $file = $files[0]; else echo 'No file with extension!'; 

or

$file = (count($files = glob('public/images/4_cover.*') != 0)) ? $files[0] : 'NO_FILE' ;

I apologize for the quite bad quality code, but that's what OP wants and that's the easiest way I could think to do that for him.

You can use the pathinfo function

$file = "file.php";
$path_parts = pathinfo($file);
$path_parts['extension']; // return => 'php'