Simplified answer is,
// PNG image url
$url = 'http://www.example.com/image.png';
// Create image from web image url
$image = imagecreatefrompng($url);
// Start output buffer
ob_start();
// Convert image
imagejpeg($image, NULL,100);
imagedestroy($image);
// Assign JPEG image content from output buffer
$content_jpg = ob_get_clean();
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