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;
}

回傳1或空

可以檢查的常數有：

[IMAGETYPE_GIF] => 1
[IMAGETYPE_JPEG] => 2
[IMAGETYPE_PNG] => 3
[IMAGETYPE_SWF] => 4
[IMAGETYPE_PSD] => 5
[IMAGETYPE_BMP] => 6
[IMAGETYPE_TIFF_II] => 7
[IMAGETYPE_TIFF_MM] => 8
[IMAGETYPE_JPC] => 9
[IMAGETYPE_JP2] => 10
[IMAGETYPE_JPX] => 11
[IMAGETYPE_JB2] => 12
[IMAGETYPE_SWC] => 13
[IMAGETYPE_IFF] => 14
[IMAGETYPE_WBMP] => 15
[IMAGETYPE_JPEG2000] => 9
[IMAGETYPE_XBM] => 16
[IMAGETYPE_ICO] => 17
[IMAGETYPE_UNKNOWN] => 0
[IMAGETYPE_COUNT] => 18

參考資料

[1] PHP best way to check if file is an image