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
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);
      }
   }
}
?>