You can't split a pixel in half. You also don't need the "px" at the end.
Do this:
<?php
$str = $_GET['txt'];
$image = imagecreatefrompng('real darkilscape userbar.JPEG');
$color = imagecolorallocate($image, 255, 255, 255);
$font = 'fonts/VISITOR.FON';
$x = 275;
$y = 9;
$size = 15 pt;
$rotation = 0;
imagefontext($image, $size, $rotation, $x, $y, $color, $font, $str);
header('Content-Type: real darkilscape userbar.JPEG');
imagepng($image);
imagedestroy($image);
?>
