The bug reported by 'michi at marel dot at' also exists in PHP version 5.1.1. This functions just works with vertical lines!

It doesn't work for horizontal lines. Use imagesetstyle()

You can use the imagepatternedline() function with extra features instead of imagedashedline() to give a visible dashedlines and other any kind of patterned lines on your images. The routine also manages the thickness of the line. Have fun!

<?php
// imagepatternedline() function
// Routine was developed to draw any kind of straight line with thickness. Routine uses imageline() function to draw line.
// Parameters are (similar to imageline() function):
//   $image: (resource) imagefile
//   $xstart, $ystart: (int) x,y coordinates for first point
//   $xend, $yend: (int) x,y coordinates for last point
//   $color: (int) color identifier that created with imagecolorallocate()
// extra parameters:
//   $thickness: (int) thickness of line in pixel
//   $pattern: (string) pattern of line, which repeats continuously while line is being drawed.
//   If there is '1' in the pattern that means the actual dot of line is visible,
//   '0' means dot is not visible (space between two line elements).
//   All characters regard for one pixel. Default: 1 dot wide dashed line with 4-4 dots and spaces.
// Examples for pattern:
// "1" or "" continuous line
// "10" close dotline
// "10000" dotline
// "111111110000001100000011111111" dotline for design drawing
// "111111111100000011000000110000001111111111" double dotline
// some examples for using imagepatternedline():
// imagepatternedline($image,300,300,442,442,$color,200,""); // a square with 200 length of edge and rotated 45 degrees
// imagepatternedline($image,100,200,289,200,$color,100, 
//  "11001100000011001111000011001111110000001100001100"
// ."00001111001100111100000011000000110000110011001100"
// ."11000011111100111111000011001111001111000011110000"
// ."1111001111110011000011000000001100110011"); // barcode 

function imagepatternedline($image, $xstart, $ystart, $xend, $yend, $color, $thickness=1, $pattern="11000011") {
   $pattern=(!strlen($pattern)) ? "1" : $pattern;
   $x=$xend-$xstart;
   $y=$yend-$ystart;
   $length=floor(sqrt(pow(($x),2)+pow(($y),2)));
   $fullpattern=$pattern;
   while (strlen($fullpattern)<$length) $fullpattern.=$pattern;
   if (!$length) {
      if ($fullpattern[0]) imagefilledellipse($image, $xstart, $ystart, $thickness, $thickness, $color);
      return;
   }
   $x1=$xstart;
   $y1=$ystart;
   $x2=$x1;
   $y2=$y1;
   $mx=$x/$length;
   $my=$y/$length;
   $line="";
   for($i=0;$i<$length;$i++){
      if (strlen($line)==0 or $fullpattern[$i]==$line[0]) {
         $line.=$fullpattern[$i];
      }else{
         $x2+=strlen($line)*$mx;
         $y2+=strlen($line)*$my;
         if ($line[0]) imageline($image, round($x1), round($y1), round($x2-$mx), round($y2-$my), $color);
         $k=1;
         for($j=0;$j<$thickness-1;$j++) {
            $k1=-(($k-0.5)*$my)*(floor($j*0.5)+1)*2;
            $k2= (($k-0.5)*$mx)*(floor($j*0.5)+1)*2;
            $k=1-$k;
            if ($line[0]) {
               imageline($image, round($x1)+$k1, round($y1)+$k2, round($x2-$mx)+$k1, round($y2-$my)+$k2, $color);
               if ($y) imageline($image, round($x1)+$k1+1, round($y1)+$k2, round($x2-$mx)+$k1+1, round($y2-$my)+$k2, $color);
               if ($x) imageline($image, round($x1)+$k1, round($y1)+$k2+1, round($x2-$mx)+$k1, round($y2-$my)+$k2+1, $color);
            }
         }
         $x1=$x2;
         $y1=$y2;
         $line=$fullpattern[$i];
      }
   }
   $x2+=strlen($line)*$mx;
   $y2+=strlen($line)*$my;
   if ($line[0]) imageline($image, round($x1), round($y1), round($xend), round($yend), $color);
   $k=1;
   for($j=0;$j<$thickness-1;$j++) {
      $k1=-(($k-0.5)*$my)*(floor($j*0.5)+1)*2;
      $k2= (($k-0.5)*$mx)*(floor($j*0.5)+1)*2;
      $k=1-$k;
      if ($line[0]) {
         imageline($image, round($x1)+$k1, round($y1)+$k2, round($xend)+$k1, round($yend)+$k2, $color);
         if ($y) imageline($image, round($x1)+$k1+1, round($y1)+$k2, round($xend)+$k1+1, round($yend)+$k2, $color);
         if ($x) imageline($image, round($x1)+$k1, round($y1)+$k2+1, round($xend)+$k1, round($yend)+$k2+1, $color);
      }
   }
}
?>

I make my own dashedline:

<?
for($l=50;$l<=550;$l+=5)
   {
    if($da == 0) { $da = 1; }
    elseif($da == 1){
    imageline($bild,$l,50,$l+5,50,$green);
    $da = 0; }
   }
?>

$l is the x-value 
and we have a dashed line :)

There's a bug till PHP 4.0.4 in this function. You can only draw vertical dashed lines. To draw other dashed lines you can set <ImageSetStyle> to a special dashed line and draw it by <ImageLine>.

Sample code:
<?php
function MDashedLine($image, $x0, $y0, $x1, $y1, $fg, $bg)
{
        $st = array($fg, $fg, $fg, $fg, $bg, $bg, $bg, $bg);
        ImageSetStyle($image, $st);
        ImageLine($image, $x0, $y0, $x1, $y1, IMG_COLOR_STYLED);
}
?>