If you would like to get the extension from a filename (string), the following should save you any frustration with exploding the string into parts (be sure to read the documentation warnings for pathinfo):

<?php

  $filename = "wallpaper.jpg";
  $extension = pathinfo( $filename, PATHINFO_EXTENSION );

  echo $extension; // "jpg"

?>

To: mail at spybreak dot de

I noted  your solution was for mime_type_to_extension which is flawed because the MIME types to extensions are not unique. See my example to consider what I have observed.

This function performs image type or mime type to extension. With limitation it will not attempt to handle duplicated MIME types. NOT DEFINITIVE!
<?php
    if(!function_exists('image_type_to_extension')){       

        $extension;

        function image_type_or_mime_type_to_extension($image_type, $include_dot) {
            define ("INVALID_IMAGETYPE", '');
            
            $extension = INVALID_IMAGETYPE;            /// Default return value for invalid input

            $image_type_identifiers = array (                                                                ### These values correspond to the IMAGETYPE constants
                    array (IMAGETYPE_GIF         => 'gif',     "mime_type" => 'image/gif'),                        ###  1 = GIF
                    array (IMAGETYPE_JPEG        => 'jpg',     "mime_type" => 'image/jpeg'),                        ###  2 = JPG
                    array (IMAGETYPE_PNG        => 'png',     "mime_type" => 'image/png'),                        ###  3 = PNG
                    array (IMAGETYPE_SWF        => 'swf',     "mime_type" => 'application/x-shockwave-flash'),    ###  4 = SWF  // A. Duplicated MIME type
                    array (IMAGETYPE_PSD        => 'psd',     "mime_type" => 'image/psd'),                        ###  5 = PSD
                    array (IMAGETYPE_BMP        => 'bmp',     "mime_type" => 'image/bmp'),                        ###  6 = BMP
                    array (IMAGETYPE_TIFF_II    => 'tiff',     "mime_type" => 'image/tiff'),                        ###  7 = TIFF (intel byte order)
                    array (IMAGETYPE_TIFF_MM    => 'tiff',     "mime_type" => 'image/tiff'),                        ###  8 = TIFF (motorola byte order)
                    array (IMAGETYPE_JPC        => 'jpc',     "mime_type" => 'application/octet-stream'),            ###  9 = JPC  // B. Duplicated MIME type
                    array (IMAGETYPE_JP2        => 'jp2',     "mime_type" => 'image/jp2'),                        ### 10 = JP2
                    array (IMAGETYPE_JPX        => 'jpf',     "mime_type" => 'application/octet-stream'),            ### 11 = JPX  // B. Duplicated MIME type
                    array (IMAGETYPE_JB2        => 'jb2',     "mime_type" => 'application/octet-stream'),            ### 12 = JB2  // B. Duplicated MIME type            
                    array (IMAGETYPE_SWC        => 'swc',     "mime_type" => 'application/x-shockwave-flash'),    ### 13 = SWC  // A. Duplicated MIME type
                    array (IMAGETYPE_IFF        => 'aiff',     "mime_type" => 'image/iff'),                        ### 14 = IFF
                    array (IMAGETYPE_WBMP        => 'wbmp',     "mime_type" => 'image/vnd.wap.wbmp'),                ### 15 = WBMP
                    array (IMAGETYPE_XBM        => 'xbm',     "mime_type" => 'image/xbm')                            ### 16 = XBM
            );                                                                                    
            
            if((is_int($image_type)) AND (IMAGETYPE_GIF <= $image_type) AND (IMAGETYPE_XBM >= $image_type)){
                $extension = $image_type_identifiers[$image_type-1]; // -1 because $image_type_identifiers array starts at [0]
                $extension = $extension[$image_type];
            }
            elseif(is_string($image_type) AND (($image_type != 'application/x-shockwave-flash') OR ($image_type != 'application/octet-stream'))){                
            
                $extension =  match_mime_type_to_extension($image_type, $image_type_identifiers);
            }
            else
            {
                $extension = INVALID_IMAGETYPE;
            }

               if(is_bool($include_dot)){

                   if((false != $include_dot) AND (INVALID_IMAGETYPE != $extension)){
                       $extension = '.' . $extension;
                   }
               }
               else
               {
                   $extension = INVALID_IMAGETYPE;
               }                  
       
           return $extension;

           }
    }    

    function match_mime_type_to_extension($image_type, $image_type_identifiers){
        // Return from loop on a match
        foreach($image_type_identifiers as $_key_outer_loop => $_val_outer_loop){            
            foreach($_val_outer_loop as $_key => $_val){
                if(is_int ($_key)){             // Keep record of extension for mime check
                    $extension = $_val;
                } 
                if($_key == 'mime_type'){    
                    if($_val === $image_type){    // Found match no need to continue looping
                        return $extension;        ### Return
                    }                     
                }
            }
        }
        // Compared all values without match
        return $extension = INVALID_IMAGETYPE;    
    } 
    
    $extension = image_type_or_mime_type_to_extension($image_type, $include_dot);
     return $extension;
}
?>

When I was writing a script for my photo website, it was necessary to write such function, which can get the extension of uploaded file (image), so the function is:

<?php
function get_extension($imagetype)
   {
       if(empty($imagetype)) return false;
       switch($imagetype)
       {
           case 'image/bmp': return '.bmp';
           case 'image/cis-cod': return '.cod';
           case 'image/gif': return '.gif';
           case 'image/ief': return '.ief';
           case 'image/jpeg': return '.jpg';
           case 'image/pipeg': return '.jfif';
           case 'image/tiff': return '.tif';
           case 'image/x-cmu-raster': return '.ras';
           case 'image/x-cmx': return '.cmx';
           case 'image/x-icon': return '.ico';
           case 'image/x-portable-anymap': return '.pnm';
           case 'image/x-portable-bitmap': return '.pbm';
           case 'image/x-portable-graymap': return '.pgm';
           case 'image/x-portable-pixmap': return '.ppm';
           case 'image/x-rgb': return '.rgb';
           case 'image/x-xbitmap': return '.xbm';
           case 'image/x-xpixmap': return '.xpm';
           case 'image/x-xwindowdump': return '.xwd';
           case 'image/png': return '.png';
           case 'image/x-jps': return '.jps';
           case 'image/x-freehand': return '.fh';
           default: return false;
       }
   }
?>

It's useful for those, who upload files on server.

// if you just want to find the format from the file path name without creating any images

// image path name
$image_name = "image name.png";

// After the last point is naturally the format.
$dots = explode(".",$image_name);

// We assign the last data of $dots to the variable $type.
$type = $dots[(count($dots)-1)];

// The output will be written without a dot.
echo "without dot : ".$type;

//for dotted
$type = ".".$type;
echo "dotted : ".$type;

2006-09-29

A few notes about some contributions on this page.

1. It seemed to me that on the face it of all of the offerings to emulate "image_type_to_extension" function fell short of the mark in one way or another (See my comments below). That's why I wrote my own and submitted to this page below. In respect of my work any comments, bugs noted or improvements would be gratefully received.

2. Avoid using the Switch statement in an unconventional method to "Break" (I note the use of the return statement!). Also even if it does nothing at the inception of our code - Still put in the default case (It lets others realise that a default is not required or at worst forgotton.

3. In an environment that is under your control the risk of an error by determining the content by its extension or MIME type may seem an attractive solution to a problem. However, in the real world there's no guarantee that a MIME type or file extension is correct for it's associated file.

Consider using functions to get the image type:
      getimagesize or (This is available without GD)
      exif_imagetype

4. There's more to coding than just putting something together to do a job!!! But whatever is done is worthwhile - Hence expletives have no place in this forum!! 

5. The idea from "oridan at hotmail dot com" is a very slick idea. I will be taking a closer look at this for my own project.