Use a brush to create a thick line.
To create a 3x3 red brush:
<?php
$brush_size = 3;
$brush = imagecreatetruecolor($brush_size,$brush_size);
$brush_color = imagecolorallocate($brush,255,0,0);
imagefill($brush,0,0,$brush_color);
imagesetbrush($im,$brush);
?>
Then use imageline() or imagepolygon() with IMG_COLOR_BRUSHED as the color.
To stop using the brush, destroy it:
<?php imagedestroy($brush); ?>
The brush can also be created from an existing image.