Generate an image via PHP

I want to generate an image via php. I've already done it with a POST request that sends a base64 code representing the content of a canvas to a php file.

JS:

var canvas = document.getElementById("myCanvas");
    var dataURL = canvas.toDataURL("image/jpeg", 0.2);
    console.log(dataURL);

    // post the dataUrl to php
    $.ajax({
        type: "POST",
        url: "upload.php",
        data: {image: dataURL}
        }).done(function( respond ) {
            // you will get back the temp file name
            // or "Unable to save this image."
            console.log(respond);
});

PHP:

<?php
if ( isset($_POST["image"]) && !empty($_POST["image"]) ) {
    $dataURL = $_POST["image"];
    $parts = explode(',', $dataURL);  
    $data = $parts[1];
    $data = base64_decode($data); 
    // create a temporary unique file name
    $file = "img/" . UPLOAD_DIR . uniqid() . '.png';
    // write the file to the upload directory
    $success = file_put_contents($file, $data);
    // return the temp file name (success)
    // or return an error message
    print $success ? $file : 'Unable to save this image.';
}
?>

I've looked to some similar sites and I've found out that they're doing the same thing with a different approach. They send a GET request to a php file, and they just send some datas (for example, the User name that has to be printed on the canvas).

My point is: speaking only about the server's performance, what is the best choice? If the best is second one, how can I achieve that result with GET? Where I've to put the canvas?