I don't think you should have been down voted for this question. It discourages asking to learn.
What I meant was mixing them together in the same piece of code. The other comments explain why: its doing too much, its difficult to maintain and reason about so its harder to find bugs, especially serious ones, and difficult to fix them.
separation of concerns and DRY would be the two main reasons I'd put forward.
Anything involving manipulating your data / business logic should not be embedded in a template.
PHP in your html should really be for display/rendering (i.e. looping over a list of Employee records), it should not be fetching those records itself (either by Employee::fetchAll() or especially mysql_query("SELECT * from employee") )
Research "model view controller" and separation of concerns.
It is very hard to maintain or even reason about a file or make a change or secure exploitable edge cases when the application has html, css, javascript, php, and calls to the database in the same place.
