PHP and MySQL Web Development 4th Ed-tqw-_darksiderg

558 Chapter 26 Debugging
Handling runtime errors requires a certain amount of forethought—to check for different
types of failure that might occur and then take appropriate action. Simulating each
class of runtime error that might occur also takes careful testing.
We do not mean that you need to attempt to simulate every different error that
might occur. MySQL, for example, can provide one of around 200 different error numbers
and messages.You do need to simulate an error in each function call that is likely to
result in an error and an error of each type that is handled by a different block of code.
Failure to Check Input Data
Often you make assumptions about the input data that will be entered by users. If this
data does not fit your expectations, it might cause an error, either a runtime error or a
logic error (detailed in the following section).
A classic example of a runtime error occurs when you are dealing with user input
data and you forget to apply addslashes() to it.This means if you have a user with a
name such as O’Grady that contains an apostrophe, you will get an error from the database
function if you use the input in an insert statement inside single quotation marks.
We discuss errors because of assumptions about input data in more detail in the next
section.
Logic Errors
Logic errors can be the hardest type of error to find and eliminate.This type of error
occurs when perfectly valid code does exactly what it is instructed to do, but that was
not what the writer intended.
Logic errors can be caused by a simple typing error, such as
for ( $i = 0; $i < 10; $i++ );
{
echo ‘doing something’;
}
This snippet of code is perfectly valid. It follows valid PHP syntax. It does not rely on
any external services, so it is unlikely to fail at runtime. Unless you looked at it very
carefully, it probably will not do what you think it will or what the programmer
intended it to do.
At a glance, it looks as if it will iterate through the for loop 10 times, echoing
“doing something” each time.The addition of an extraneous semicolon at the end of
the first line means that the loop has no effect on the following lines.The for loop will
iterate 10 times with no result, and then the echo statement will be executed once.
Because this snippet is a perfectly valid, but inefficient, way to write code to achieve
this result, the parser will not complain. Computers are very good at some things, but
they do not have any common sense or intelligence. A computer will do exactly as it is
told.You need to make sure that what you tell it is exactly what you want.