how can I disable php file upload in wordpress

[admin]

I am trying to make upload avatar image for wordpress users in front end. I am using this code

$files = $_FILES['post_files'];

foreach($files as $file){

  if(is_array($file)){

    $uploaded_file_type = $file['type'];
    $allowed_file_types = array('image/jpg', 'image/jpeg', 'image/png', 'image/gif');
    if(!in_array($uploaded_file_type, $allowed_file_types)) {
      $errors['image_empty'] = __( 'this image is not valid', 'themename' );
    } 

  }

}

it's not allowing to upload php file , but if they change the php file extension as png or jpeg they can upload the php file to my server. I try to use

getimagesize()

but I couldn't, I am newbie for php. Or is there any other solution?

thanks for answers