PHP Draw sinewave with ImageGD

<?php
// Draws sinewave in PHP
// Author: fastman92
// Site: fastman92-site.tk

// Input data
$sizeX = 600;
$sizeY = 300;
$period = 200;    // px

// Code
$im = imagecreatetruecolor($sizeX, $sizeY);
$white = imagecolorallocate($im, 255, 255, 255);

imagefilledrectangle($im, 0, 0, $sizeX, $sizeY, $white);

$color = imagecolorallocate($im, 0, 0, 0);

$previousY = 0.0;

    for($x = 0; $x < $sizeX; $x++)
    {
        $y = ($sizeY / 2.0) + $sizeY/2 * sin(2*M_PI*$x/$period);
        imageline($im, $x-1, $previousY, $x, $y, $color);

        $previousY = $y;
    }

header('Content-Type: image/png');
imagepng($im);

imagecolordeallocate($im, $color);
?>