Description:
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https://github.com/php/php-langspec/blob/master/spec/04-basic-concepts.md#program-structure

The "<?php" open tag doesn't have to be at the top of the file, it can come later, and technically a PHP file might not have any tags at all. Also, after the "?>" close tag there can be more stuff and possible another open tag "<?php". Finally, there are some interesting things like how "<?" (with the "php" part) work as an open tag, and how "<script language='php'>" can also work as an open tag. Stuff that appears before the first open tag and stuff that appears after a close tag but before the next open tag is treated as content that is echo'd out when the flow of execution reaches it. 

Some of this has been addressed by changing the grammar for the syntactic grammar production “program”.