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

Programming Errors
557
A SQL query that is syntactically valid and refers only to databases, tables, and
columns that exist generally does not fail.The query might, however, return no results if
it is querying an empty table or searching for data that does not exist. Assuming that you
have connected to a database successfully and have a table called t1 and a column called
c1, the query
select * from t1 where c1 = ‘not in database’;
will succeed but not return any results.
Before you use the result of the query, you need to check for both failure and no
results.
Connections to Network Services
Although devices and other programs on your system will occasionally fail, they should
fail rarely unless they are of poor quality.When using a network to connect to other
machines and the software on those machines, you need to accept that some part of the
system will fail often.To connect from one machine to another, you rely on numerous
devices and services that are not under your control.
At the risk of our being repetitive, you really need to carefully check the return value
of functions that attempt to interact with a network service.
A function call such as
$sp = fsockopen('localhost'’, 5000 );
will provide a warning if it fails in its attempt to connect to port 5000 on the machine
localhost, but it will display it in the default format and not give your script the option
to handle it gracefully.
Rewriting the call as
$sp = @fsockopen ('localhost', 5000, &$errorno, &$errorstr );
if(!$sp) {
echo "ERROR: ".$errorno.": ".$errorstr;
}
will suppress the built-in error message, check the return value to see whether an error
occurred, and use your own code to handle the error message. As the code is written, it
will display an error message that might help you solve the problem. In this case, it
would produce the following output:
ERROR: 10035: A non-blocking socket operation could not be completed immediately.
Runtime errors are harder to eliminate than syntax errors because the parser cannot signal
the error the first time the code is executed. Because runtime errors occur in
response to a combination of events, they can be hard to detect and solve.The parser
cannot automatically tell you that a particular line will generate an error.Your testing
needs to provide one of the situations that create the error.