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Creating invoices in PDF is always a bit tricky: there are many libraries to create PDF documents with PHP, but most can't handle complex layouts and require a lot of memory and CPU time. Things like Unicode characters and line/page breaks are often difficult to program and the source of many bugs and memory problems. Using logos as vector graphics or embedding TrueType fonts is often required, but not possible with most libraries. The good thing is that all these features are included in HTML5. Since most companies offer their invoices also in HTML, it would be great to convert them directly to PDF. Tools like html2ps and ps2pdf can produce PDFs with 200+ pages without problems, but only support HTML4 and some very limited CSS. Also, the layout is never the same as in a web browser. So we need a real web browser to convert HTML pages to PDFs. From testing web applications, we know that PhantomJS is a headless WebKit browser that runs on the commandline and can automate things ...

Lessons learned: A web server written in PHP can give more than 10000 req/s on small hardware A web server written in PHP is not slower than being written in Java (without threads) A web server written in PHP is 30 percent slower than being written in C (without threads) Realtime applications can be developed in PHP without problems PHP normally runs inside a web server like Apache or nginx. This keeps all requests separate from each other and does not allow sharing memory or connections. To implement a chat server, the browser has to poll the server regularly for new data. The data is stored in a database and looked up for each request. This is very slow, takes a lot of resources on the server and does not give messages in realtime. Newer browsers support data being pushed from the server to the client. There are two techniques used: WebSockets (full-duplex) and Server-Sent Events (push notifications) Using these techniques, one connection stays open for each client and the server...