Operators
Computers control the flow of electrons in circuits that are either on or off. Controlling this flow gives rise to how a computer works, and how computer languages work. We say that a program is running, because even if it does nothing on the screen, the flow is cycling, waiting for something to trigger a change in the flow.
Operators manipulate this flow, using simple logic and math in creative ways. Basic logic structures the flow and determines how the result is to be assigned; if this is equal to that or this and that. There are boolean operators that signify the outcome of the flow as true or false, causal statements, that say if condition is met, do that, or else it redirects the flow in another direction, and lots of mathematical operators that go way beyond addition and subtraction.
PHP's use is primarily connected up with databases that store the snippets of content that PHP can pre-process into HTML pages. To do that it spends a lot of the time talking to databases and requesting and looping through the content to organize it into HTML pages. We are not going to go any further into the PHP language itself, but know that many of the larger sites use this kind of server side scripting.