(PHP 5 >= 5.2.0, PHP 7, PHP 8)
image_type_to_extension — Get file extension for image type
   Returns the extension for the given IMAGETYPE_*
   constant.
  
image_type
       One of the IMAGETYPE_* constant.
      
include_dot
       Whether to prepend a dot to the extension or not. Default to true.
      
   A string with the extension corresponding to the given image type,  or false on failure.
  
Example #1 image_type_to_extension() example
<?php
// Create image instance
$im = imagecreatetruecolor(100, 100);
// Save image
imagepng($im, './test' . image_type_to_extension(IMAGETYPE_PNG));
?>Note:
This function does not require the GD image library.