Validate that a file is a picture in PHP

best way to check if file is an image

function is_image($path)
{
    $a = getimagesize($path);
    $image_type = $a[2];

    if(in_array($image_type , array(IMAGETYPE_GIF , IMAGETYPE_JPEG ,IMAGETYPE_PNG , IMAGETYPE_BMP)))
    {
        return true;
    }
    return false;
}

more: http://www.binarytides.com/php-check-if-file-is-an-image/

The most efficient way would be to look at the beginning bytes of the file and test for 'magic number' file specifier. Here is a list of magic numbers.

Using (part) of the GD library.

PHP: GD - Manual

array getimagesize ( string $filename [, array &$imageinfo ] )

The first element of the array will be 0 if there is no image. PHP: getimagesize

If you don't have GD installed (most of the time you will), you can read the file header as Shane mentioned.

EDIT: Actually, as Neal pointed out in the comments, the GD library is not even required to use this function. So use it.