The Collatz Conjecture (Graphical Output)

<?php
function collatz($originalNumber){ //the collatz conjecture function
  $number = $originalNumber;
  do{
  $i++;
      if(1&$number){
        $number = (3*$number)+1;
      }else{
        $number = $number/2;
      }
  }while($number != 1);
  return $i;
}

echo "Collatz Conjecture\n"; // :D

$n = 1;
do{ //could've used a for loop, but I think this worked out better.
  $i = collatz($n); //testing how many steps that it will take for the number $n to reach 1.
  if($i > $maxheight){ //if the result is larger than all the  rest, the new result will become the largest result.
    $maxheight = $i;
  }
  $x[$n] = $i; //$n is the position along the x-axis, $i is the position along the y-axis.
  $n++; //next number
}while($n < 1000); //find the amount of steps that it takes for numbers 1 to one thousand... which is probably a little high. CHANGE THIS DEPENDING ON HOW MANY NUMBERS YOU WANT TO TEST

//---begin writing file
unlink("image.ppm"); //file that we will be editing, we are are deleting the file so we have a clean file to work on
$fh = fopen("image.ppm", 'a+'); //creates the image file that we will be editting and opens the file for appending

$width = $n; //width of the image is the largest number that was tested
$height = $maxheight; //height is the largest amount of steps that it took to reach zero

$startWrite = "P3\n# image.ppm\n".$width." ".$height."\n255\n"; //fomatting the ppm file
fwrite($fh, $startWrite);

$Nl = " ";
for($k = 0; $k < $width*$height; $k++){
  $CurPx = $k%$n; //basic modulus to decide when a new line must be made
  if($CurPx == 0){ //^ditto^
    $CurPy++;
  }
  if($x[$CurPx] == $height-$CurPy){ //basic algebra
    $out = "255"; //if the if statement above reflects the array which was made on line 23, the pixel will be white
  }else{
    $out = "000"; //else, it will be black
  }
  $write = "$out $out $out$Nl"; //the line that is to be outputted
    fwrite($fh, $write); //write the pixel to the file.
    if($CurPx%5 == 0){ //basic formatting decides when a new line must be placed
    $Nl = "\n";
  }else{
    $Nl = " ";
  }
}
fclose($fh); //close the file, done!
?>