(PHP 4 >= 4.3.0, PHP 5)
image_type_to_mime_type — Get Mime-Type for image-type returned by getimagesize, exif_read_data, exif_thumbnail, exif_imagetype
The image_type_to_mime_type() function will determine the Mime-Type for an IMAGETYPE constant.
One of the IMAGETYPE_XXX constants.
The returned values are as follows
| imagetype | Returned value | 
|---|---|
| IMAGETYPE_GIF | image/gif | 
| IMAGETYPE_JPEG | image/jpeg | 
| IMAGETYPE_PNG | image/png | 
| IMAGETYPE_SWF | application/x-shockwave-flash | 
| IMAGETYPE_PSD | image/psd | 
| IMAGETYPE_BMP | image/bmp | 
| IMAGETYPE_TIFF_II (intel byte order) | image/tiff | 
| IMAGETYPE_TIFF_MM (motorola byte order) | image/tiff | 
| IMAGETYPE_JPC | application/octet-stream | 
| IMAGETYPE_JP2 | image/jp2 | 
| IMAGETYPE_JPX | application/octet-stream | 
| IMAGETYPE_JB2 | application/octet-stream | 
| IMAGETYPE_SWC | application/x-shockwave-flash | 
| IMAGETYPE_IFF | image/iff | 
| IMAGETYPE_WBMP | image/vnd.wap.wbmp | 
| IMAGETYPE_XBM | image/xbm | 
| IMAGETYPE_ICO | image/vnd.microsoft.icon | 
Beispiel #1 image_type_to_mime_type() example
<?php
header("Content-type: " . image_type_to_mime_type(IMAGETYPE_PNG));
?>
Hinweis:
Diese Funktion benötigt die GD-Bibliothek nicht.
I was looking to detect the format of images, Im not sure if this is the best way but it works for me.
<?php
$filename = 'http://static.php.net/www.php.net/images/php.gif';
$size = getimagesize($filename);
switch ($size['mime']) {
    case "image/gif":
        echo "Image is a gif";
        break;
    case "image/jpeg":
        echo "Image is a jpeg";
        break;
    case "image/png":
        echo "Image is a png";
        break;
    case "image/bmp":
        echo "Image is a bmp";
        break;
}
?>
This functionality is now implemented in the PEAR package PHP_Compat.
More information about using this function without upgrading your version of PHP can be found on the below link:
http://pear.php.net/package/PHP_Compat