Watch out! If you pass imagesetstyle() an empty array as the second argument, it will crash your server!
I was messing with it just earlier and accidentally did so, and the page took a good minute to process, when my Apache server came up with the good ol' Windows 'Send Error Report' window.

To clarify, for lines where the thickness is greater than 1, the total length of the $style array needs to be an exact divisor of the length of the line.

This is because the pattern is repeated lengthways and wraps onto the second row of pixels, causing staggering to occur.

So if you have 5 red and 5 white pixels, and you want a line length of 55 pixels, either change the length to a multiple of 10, or change the dashes to, say, 6 red and 5 white.

a shortcut for basic dashed lines, making it easy to adjust the lengths:

<?php

$length1 = 20;
$length2 = 10;
$style = array_merge(array_fill(0, $length1, $red), array_fill(0, $length2, $w));
imagesetstyle($im, $style);

?>

When lines drawn with imagesetstyle seem to produce a thin white line only, make sure antialiasing is disabled.

<?
  imageantialias($im, false);
  $style = array($gridxcolor, $gridxcolor, IMG_COLOR_TRANSPARENT, IMG_COLOR_TRANSPARENT);
  imagesetstyle($im, $style);
  imageline($im, $x, 0, $x, $ymax+5, IMG_COLOR_STYLED);
  imageantialias($im, true);
?>

Setstyle and Antialias don't go together.

Be aware that styles are applied towards the width of the line instead of linear.
To convert a style to be used for thick lines you can use the function below:

<?php
/*
ImageStyleThicken(<aStyle>,<iThickness>) --> <aThickStyle>

<aStyle> is the style array for a thickness of 1 (see imagesetstyle()).
<iThickness> is the new thickness to apply (see imagesetthickness()).

<aThickStyle> is the style array suitable for the given thickness.
*/
function ImageStyleThicken($_1,$_2) {
    $a = array();
    foreach ($_1 as $x) {
        $i = $_2;
        do $a[] = $x; while (--$i>0); }
    return $a;
}
?>

Function to make a line with random fading:

<?php
    function fading_line($img,$sx,$sy,$ex,$ey){
        $r=rand(0,5);$g=rand(0,5);$b=rand(0,5);
        $l=sqrt((($ex-$sx)*($ex-$sx))+(($ey-$sy)*($ey-$sy)));
        for($i=0;$i<$l;$i++){
            $a = array(255-((255/$l)*$i), 255,0,(255/$l)*$i/2,(255/$l)*$i,(255-((255/$l)*$i))/2);
            $style[]=imagecolorallocate($img,$a[$r],$a[$g],$a[$b]);
        }
        imagesetstyle($img,$style);
        imageline($img,$sx,$sy,$ex,$ey,IMG_COLOR_STYLED);
    }
    
    fading_line($img,10,20,490,40);    // image, start x, start y, end x, end y
?>

Use this to set the style to any combination of pixels.
You can pass as many modifiers as you wish.
Use the format [num]r[red]g[green]b[blue].

For example:

$im=dashed($im,"4r255g0b0","2r0g255b0","5r0g0b255");
imageline($im, 0, 0, 600, 600, IMG_COLOR_STYLED);

function dashed($im){
$size = func_num_args();
for($i = 0; $i < $size; $i++){
$arg = func_get_arg($i);
if(!is_resource($arg)){
$r=substr($arg,strpos($arg,"r")+1,
strpos($arg,"g")-strpos($arg,"r")-1);
$g=substr($arg,strpos($arg,"g")+1,
strpos($arg,"b")-strpos($arg,"g")-1);
$b=substr($arg,strpos($arg,"b")+1,
strlen($arg)-strpos($arg,"b"));
$color = imagecolorallocate($im,$r,$g,$b);
$x = substr($arg,0,strpos($arg,"r"));
$vals[$i] = array_fill(0,$x,$color);
}
}
for($k=0;$k<count($vals)+1;$k++)
if(array_key_exists($k,$vals)) $prop=array_merge($prop,$vals[$k]);
imagesetstyle($im, $prop);
return $im;
}