Good example, but in the case of PHP, emitting the ending tag is completely acceptable by the standard and is a common best practice as well, especially if your final content in the file is PHP code and not, for example, HTML.

I think what the you're suggesting is more like stretching the rules and perhaps going outside of the standard a little, but in a way that most implementations would be able to handle.

Right, non valid, strictly speaking, yet perfectly parsed by most interpreters. That’s where the magic happens

I could see implementations tolerating a lack of DEFLATE Adler-32, lack of PNG chunk CRC-32, and lack of PNG IEND chunk.