PHP Template: Image file upload handler

Question

I'm trying to develop some templates for common PHP tasks I've been dealing with. One of which is a general image file upload handler.

So far I'm using the following reusable code which seems to be working fine without any noticeable bug:

<?php

    if ( !isset($_POST['submit']) ) {
        goto page_content;}

    if ( $_FILES['file_upload']['error']===4 ) {
        echo 'No file uploaded';
        goto page_content;}

    if ( $_FILES['file_upload']['error']===1 || $_FILES['file_upload']['error']===2 ) {
        echo 'File exceeds maximum size limit';
        goto page_content;}

    if ( $_FILES['file_upload']['error']!==0 ) {
        echo 'Failed to upload the file';
        goto page_content;}

    if ( !is_uploaded_file($_FILES['file_upload']['tmp_name']) ) {
        echo 'Failed to upload the file';
        goto page_content;}

    require_once('imageResize.php');
    $err = imageResize($_FILES['file_upload']['tmp_name'], 'random.png' );
    if ( $err !== 0 ) {
        echo 'Invalid image format';
        goto page_content;}

    echo 'Image uploaded successfully';

    page_content:
?>
<form action="filename.php" method="POST" enctype="multipart/form-data">

    <input type="hidden" name="MAX_FILE_SIZE" value="1000000">
    <input type="file" name="file_upload" accept="image/*">
    <input type="submit" name="submit">

</form>

Additional file imageResize.php:

<?php
    // image resize
    function imageResize($source, $target){

        $size = getimagesize($source);
        if ($size === false) {return 1;} // invalid image format

        $sourceImg = @imagecreatefromstring(@file_get_contents($source));
        if ($sourceImg === false) {return 2;} //invalid image format

        $width = imagesx($sourceImg);
        $height = imagesy($sourceImg);
        $sidelenght = min($width,$height);
        $targetImg = imagecreatetruecolor(100, 100);
        imagecopyresampled($targetImg, $sourceImg, 0, 0, ($width-$sidelenght)/2, ($height-$sidelenght)/2, 100, 100, $sidelenght, $sidelenght);
        imagedestroy($sourceImg);
        imagepng($targetImg, $target);
        imagedestroy($targetImg);

        return 0;           
    }
?>

Some main characteristics of this code are:

• provides messages for the most common errors that can happened during the upload process
• it allows the client to upload an image file up to 1Mb size
• resizes all images to a standard 100x100 px size
• save all images to a standard PNG format

Questions

1. Is this code safe? Or are there any vulnerability that could be exploited by an malicious client? In this case, how can I solve it?
2. To avoid several nested if-then-else conditions (which can become hard to read), I'm currently using goto (which can become a bad control structure practice). Is there a better alternative?

Answer 1

DO NOT use GOTO. They have been criticised since 1960s.

1. Your indentation is broken for the first snippet. It is also inconsistent with your second snippet. Keep it consistent throughout a project. Try looking at some linters or follow a style guide.
2. Do not suppress your errors by using @. Let the function throw any and all errors it encounters. You should instead make use of try-catch blocks.
3. While you could replace the error lookups by error code to a switch-case block, I'd suggest that you use an associated array (or a hashtable) to keep a mapping from ERROR_CODE => "ERROR MESSAGE". This will go in a separate errors.php maybe, and referenced as needed.
4. Define the target image resolution as a constant, instead of placing magic numbers.
5. You could also have a client side javascript snippet for checking file size, so that users do not waste their bandwidth and time waiting for upload process.