There are different categories of an error message that the PHP interpreter may fall into:
PARSE ERROR: usually problems with the syntax of the program. This includes forgetting to place a semi-colon by the end of a statement. The interpreter will stop running when it encounters a parse error.
FATAL ERROR: this means a severe problem with the program content. Like in a parse error, the interpreter will also stop running when encountering a fatal error.
WARNING: an advisory that the interpreter gives whenever there is something fishy in your program. But unlike the first two errors, the interpreter will still keep on running.
NOTICE: this is a tip from the PHP interpreter.