sessioncommunities.online/sites/+components/qr_modals.php

<?php
	function room_qr_code_cached($room_id) {
		global $QR_CODES;
		return "$QR_CODES/$room_id.png";
	}

	 /*
	 * Use Google API to generate QR codes and encode them as base64
	 */
	function base64_qr_code($room_id, $join_url, $size = "512x512") {
		// Could use http_build_query() instead, but I won't break what works.  
		// https://developers.google.com/chart/infographics/docs/qr_codes
		$png_cached = room_qr_code_cached($room_id);
		if (file_exists($png_cached)) 
//			fwrite(STDERR, "QR code found for " . $room_id . PHP_EOL);
			return base64_encode(file_get_contents($png_cached));
//			fwrite(STDERR, "QR code NOT found for " . $room_id . PHP_EOL);
		$data = urlencode($join_url);
		$api_url =
			"https://chart.googleapis.com/chart?cht=qr" .
			"&chs=$size" .
			"&chl=$data" .
			"&chld=L|0"; 
			// error correction level: L = 7%, M = 15%, Q = 25%, H = 30% 
			// | margin in number of rows
		$png = file_get_contents($api_url);
		file_put_contents($png_cached, $png);
		return base64_encode($png);
	}
	
	file_exists($QR_CODES) or mkdir($QR_CODES, 0700);
?>

<?php foreach ($rooms as $id => $room): ?>
<div id="modal_<?=$id?>" class="qr-code-modal">
	<div class="qr-code-modal-content">
		<span class="qr-code-modal-close" onclick='hideQRModal("<?=$id?>")'>
			&times;
		</span>
		<img 
			src="data:image/png;base64,<?=base64_qr_code($id, $room->join_link)?>"
			alt="Community join link encoded as QR code"
			class="qr-code"
		>
	</div>
</div>
<?php endforeach; ?>