PHP image loop throws HTTP 500 error

I'm trying to use PHP to output text strings as images ( a security measure) in a loop.
It works fine for up to 10,000 iterations but throws the wretched 500 Internal Server Error when I try 20,000.

It's like there's a memory leak but I've pared the code back and still have no idea what's wrong.
Here's an example - can anyone else try it?
Note - I've removed the code that uses the csv.png file containing the text as an image

<?php


function OutputToDataURI($text) {
$im = imagecreate(550,18);
$white = imagecolorallocate($im, 255, 255, 255);
$black = imagecolorallocate($im, 0, 0, 0);
imagefilledrectangle($im, 0, 0, 550, 18, $white);
imagestring($im, 3, 1, 1, $text, $black);
$imgdata = "csv.png";
imagepng($im,$imgdata,0,NULL);
imagecolordeallocate($im,$white);
imagecolordeallocate($im,$black);
imagedestroy($im);
// do something with the file csv.png here
}

$n = 0;
While($n < 20000)
{
$n = $n+ 1;
echo "<br>$n";
$text = "Hello $n";
OutputToDataURI($text);
}


echo "<p>Done n= $n";
exit();


?>



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