PHP Image Upload with Validation and Security Checks

<?php

if(isset($_FILES["fileToUpload"])) {
    $target_dir = "uploads/";
    $target_file = $target_dir . basename($_FILES["fileToUpload"]["name"]);
    $imageFileType = strtolower(pathinfo($target_file,PATHINFO_EXTENSION));
    
    // Check if image file is a actual image or fake image
    if(isset($_POST["submit"])) {
        $check = getimagesize($_FILES["fileToUpload"]["tmp_name"]);
        if($check !== false) {
            echo "File is an image - " . $check["mime"] . ".";
            $uploadOk = 1;
        } else {
            echo "File is not an image.";
            $uploadOk = 0;
        }
    }
    
    // Check if file already exists
    if (file_exists($target_file)) {
        echo "Sorry, file already exists.";
        $uploadOk = 0;
    }
    
    // Check file size
    if ($_FILES["fileToUpload"]["size"] > 500000) {
        echo "Sorry, your file is too large.";
        $uploadOk = 0;
    }
    
    // Allow certain file formats
    if($imageFileType != "jpg" && $imageFileType != "png" && $imageFileType != "jpeg"
    && $imageFileType != "gif" ) {
        echo "Sorry, only JPG, JPEG, PNG & GIF files are allowed.";
        $uploadOk = 0;
    }
    
    // Check if $uploadOk is set to 0 by an error
    if ($uploadOk == 0) {
        echo "Sorry, your file was not uploaded.";
    // if everything is ok, try to upload file
    } else {
        if (move_uploaded_file($_FILES["fileToUpload"]["tmp_name"], $target_file)) {
            echo "The file ". htmlspecialchars( basename( $_FILES["fileToUpload"]["name"])). " has been uploaded.";
            $image = imagecreatefromstring(file_get_contents($target_file));
            
            $width = imagesx($image);
            $height = imagesy($image);
            $pixel_size = 5; // Set pixel size
            
            // Create canvas for new image
            $new_image = imagecreatetruecolor($width * $pixel_size, $height * $pixel_size);
            
            // Loop through each pixel and generate a random color
            for ($x = 0; $x < $width; $x++) {
                for ($y = 0; $y < $height; $y++) {
                    $rgb = imagecolorat($image, $x, $y);
                    $r = ($rgb >> 16) & 0xFF; // Extract the red component
                    $g = ($rgb >> 8) & 0xFF; // Extract the green component
                    $b = $rgb & 0xFF; // Extract the blue component
                    $color = imagecolorallocate($new_image, rand(0, 255), rand(0, 255), rand(0, 255)); // Generate a random color
                    imagefilledrectangle($new_image, $x * $pixel_size, $y * $pixel_size, ($x + 1) * $pixel_size - 1, ($y + 1) * $pixel_size - 1, $color);
                }
            }
            
            // Save new image
            imagepng($new_image, 'output.png');
            
        } else {
            echo "Sorry, there was an error uploading your file.";
        }
    }
}

<!DOCTYPE html>
<html>
<body>

<form action="" method="post" enctype="multipart/form-data">
    Select image to upload:
    <input type="file" name="fileToUpload" id="fileToUpload">
    <input type="submit" value="Upload Image" name="submit">
</form>

</body>
</html>

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PHP Image Upload with Validation and Security Checks