You want to use the gd library for this. Here's an example which will take a png image and output a jpeg one. If the image is transparent, the transparency will be rendered as white instead.
<?php
$file = "myimage.png";
$image = imagecreatefrompng($file);
$bg = imagecreatetruecolor(imagesx($image), imagesy($image));
imagefill($bg, 0, 0, imagecolorallocate($bg, 255, 255, 255));
imagealphablending($bg, TRUE);
imagecopy($bg, $image, 0, 0, 0, 0, imagesx($image), imagesy($image));
imagedestroy($image);
header('Content-Type: image/jpeg');
$quality = 50;
imagejpeg($bg);
imagedestroy($bg);
?>