There are many way to check image extension before uploading.
But "in_array()" function it is very useful way to check image extension when you are uploading a lots of extensions to validate.

if the "in_array()" function fails, this mean condition is failed then.

You can see below example that how you can check image extension before uploading a image ?

<?php
 //here $image_type is a array variable  
  $image_type = array('gif','jpg','jpeg','png');

   $upload_image = 'newimage.png';
    // here  Utilizing strtolower to surmount case sensitive
   $extension = strtolower(pathinfo($upload_image, PATHINFO_EXTENSION));

 //here call in_array() to check image extension 
  if (in_array($extension, $image_type))

   {

     echo "image extension is valid";

   }

  else 

  {

     echo 'image extension not valid';

  }

    ?>

Output will come following:
when image extension exist in "in_array()" function
then, result will be as shown below

image extension is valid

otherwise, result will be

image extension not valid