Checks whether a given resource is an image by creating an image with the corresponding PHP-Image Functions

is_image.php

<?php
/**
 * Checks whether a given resource is an image by creating an temporary 
 * image with the corresponding PHP-Image Functions. Set a custom Extension 
 * through the second parameter to disable auto-detection and test any file. 
 * Enable "fopen wrappers" to check also URLs
 *
 * @param String $file to check. To check an uploaded file use: $_FILES['tmp_name'][X]
 * @param String $ext set file Extension to check any File
 * @return Bool
 */
function isImage(String $file, String $extension=null): Bool
{
    $useFn = [
        'gif' => '\imagecreatefromgif',
        'png' => '\imagecreatefrompng',
        'webp' => '\imagecreatefromwebp',
        'jpg' => '\imagecreatefromjpeg',
        'jpeg' => '\imagecreatefromjpeg',
    ];
    return $createFrom = ($useFn[\strtolower($extension ?? \pathinfo($file, \PATHINFO_EXTENSION))] ?? false) AND @$createFrom($file);
}

$img = 'https://upload.wikimedia.org/wikipedia/commons/1/17/Faust_image_19thcentury.jpg';
if (isImage($img)) {
    print "<img src="$img" alt="Detected Image, {$img}">";
} else {
    print 'File is not an image';
}