Able to bypass Image validation

• Laravel Version: 5.4
• PHP Version: 5.6.24

I have wrapped an executable inside an image which has the following content -

GIF89a<?php
echo 'hacked';
?>

and saved it as filename image.gif.

Now when I upload this image then it's able to bypass the image and dimension validation.

'photo' => 'required|image|dimensions:min_width=20,min_height=20',

Is there a way to validate this kind of images?

A workaround I have tried -

Used PHP function getimagesize() but it returns

array:6 [▼
  0 => 16188
  1 => 26736
  2 => 1
  3 => "width="16188" height="26736""
  "channels" => 3
  "mime" => "image/gif"
]

although image is not valid but still it has valid width and height.