How to draw sine and cosine curves on pictures in php

/**
* How to draw sine and cosine curves on pictures
* edit: bbs.it-home.org
*/
 define("MAX_WIDTH_PIXEL", 600);
 define("MAX_HEIGHT_PIXEL", 240);
 //Send header information
 header("Content -type: image/gif");
 //Create image
 $img = imageCreate(MAX_WIDTH_PIXEL, MAX_HEIGHT_PIXEL);
 //Set color
 $bgcolor = imageColorAllocate($img, 0xff, 0xe9, 0xe9);
 $red = imageColorAllocate($img, 255, 0, 0);
 $blue = imageColorAllocate($img, 0, 0, 255);
 $brown = imageColorAllocate($img, 100, 0, 0);
 $black = imageColorAllocate ($img, 0, 0, 0);
 $width = MAX_WIDTH_PIXEL/2; //Width
 $height = MAX_HEIGHT_PIXEL/2; //Height
 //Establish coordinate axis
 imageLine($img, $width, 0, $width, MAX_HEIGHT_PIXEL, $black);//y-axis
 imageLine($img, 0, $height, MAX_WIDTH_PIXEL, $height, $black);//x-axis
 //Depicting function graphics through loops
 for ($i=0; $i<=MAX_WIDTH_PIXEL; $i++)
 {
 $y1 = 100 * sin($i/100 * M_PI);
 imageSetPixel($img, $i, $height+$y1, $blue) ;
 $y2 = 100 * sin($i/300 * M_PI);
 imageSetPixel($img, $i, $height+$y2, $red);
 $y3 = 100 * sin($i/300 * M_PI) ;
 imageSetPixel($img, $i, $height-$y3, $brown);
 }
 //Display graphics
 imageGif($img);
 //Release resources
 imageDestroy($img);
?>