Run-time errors are slightly worse than syntax errors because they are not quite as obvious or easy to pin down. There are quite a few different ways in which a run-time error can occur, and many of these can only be caught by thorough testing on your live site as well as your development machine. For example, file paths may change between your development machine and your live site, breaking links, losing files, or disrupting connections. While these errors won’t show up when the PHP script is parsed, they will show up when it is executed.
Logic errors are quite sneaky in that they won’t really show up at all. It is incumbent on you to decide whether your site is performing as it should. For example, accidentally typing a + sign instead of a - won’t show up as either a run-time error or a syntax error, but will obviously affect the result of a calculation, which can have disastrous effects if it is part of your invoice calculator.
Unfortunately, there are quite a few more miscellaneous errors that can rear their ugly heads at inopportune moments, which we won’t cover in much detail here. It is important at this stage to recognize that errors will creep into your application at some point and that you will need to deal with them effectively if you are to be successful. Appendix A, which deals with problems, discusses various tactics for hunting down and removing errors, as well as looking over some of the more common errors and problems that arise while using osCommerce. One thing that should give you some comfort at this stage is that since osCommerce handles so much of the underlying code for us, the number of errors you encounter should be a bare minimum.