A function is, generally, a single word with capitalisation to show where two words were put together. e.g:
Some functions accept “arguments” inside the brackets which will alter the output, transform your input, etc. E.g:
Each function behaves differently and so you should look it up on the PHP manual page
This is the full function description for the “date” function at the top of the page:
The “$format” part says that string must be in the form of the format specified lower down the page. (note: the variable named here is the name of the section relevant to it – so $format has a format section on the manual page)
Anything in square brackets is an optional argument.
“int” refers to the type of the input that is acceptable.
“$timestamp” is the same as above and refers to a section further down the manual page.
“$timestamp = time()” says that the default value (if $timestamp is left out) is whatever the output of the “time” function is.