There are three types of errors in PHP.
1. Notice: These are very normal and non-critical errors and they even not displayed to users. For examples accessing a variable that not existed at all.
2. Warning: These are little bit critical errors. These results are displayed to users but do not affect the output of the code. For example, trying to include any file that does not exist.
3. Fatal errors: These are really critical errors. For example calling any non existence class.