How can I use ImageMagick (with the php extension) to set the transparent background to white when converting an image from PNG to JPEG?
At time of writing, you have not specified which extension you are using, but if you were using the commandline, the command would be:
convert image.png -background white -flatten -alpha off image.jpg
More information can be found on the Masking Usage documentation.
Using IMagick for instance, I think you could do this as follows:
(totally untested, never used IMagick and don't have it installed to test)
$image = new IMagick('image.png');
$flattened = new IMagick();
$flattened->newImage($image->getImageWidth(), $image->getImageHeight(), new ImagickPixel("white"));
$flattened->compositeImage($image, imagick::COMPOSITE_OVER, 0, 0);
$flattened->setImageFormat("jpg");
$flattened->writeImage('image.jpg');
$image->clear();
$image->destroy();
$flattened->clear();
$flattened->destroy();