I think your problem is here:
$font = "ARIAL.TFF";
in fact, on your server a file named "ARIAL.TFF" doesn't exist, the font extension you are looking for is TTF, not TFF, and in fact "ARIAL.TTF" on your server exists and i just downloaded it, correct that line to:
$font = "ARIAL.TTF";
After that, you should be able to write text on your image
Hope this helps
UPDATE
After question update, i noticed that an header is sent after printing out some text.
Nothing has to be printed out before the header()
function, in order for headers to be correctly sent.
In the PHP manual>header() it is the first thing that is explained:
Remember that header() must be called before any actual output is sent, either by normal HTML tags, blank lines in a file, or from PHP. It is a very common error to read code with include, or require, functions, or another file access function, and have spaces or empty lines that are output before header() is called. The same problem exists when using a single PHP/HTML file.