Php Error Traps
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the NYPHP flickr Group. Link up with NYPHP at the PHP LinkedIn Group. About NYPHP » Charter » Mission » What is NYPHP? » Principals » Sponsors PHP Error Handling NYPHP - PHundamentals PROBLEM: Every well-constructed PHP application should have http://nyphp.org/PHundamentals/7_PHP-Error-Handling error handling. While there is no definitive method for handling errors since it varies depending on application needs and a developer's style, nonetheless, there are some "best practices" that should be implemented in all PHP applications. WHAT http://linuxcommand.org/wss0160.php IS AN ERROR? The answer may seem straightforward but actually requires a bit of discussion. An "error" is an expected or unexpected event that occurs when your PHP code is running. An "expected" error can be a database php error query that returns no result or an html form that is missing values for designated required fields. An "unexpected" error is one that assumes a particular application state which, for some as yet unknown reason, does not exist. The most obvious unexpected error is a database that is not running or a missing file that one of your scripts expects to find. ERROR HANDLING ENVIRONMENT The recommended "best practice" has three parts: The developer should be php error handling alerted to all errors, notices, warnings, etc. both during development and when the PHP application goes into production. The developer should be able to choose how to be notified of these problems. No errors, notices, warnings, etc. should ever be displayed to the user. Even in development, errors dumped to the browser can be bad because they can become hidden in the HTML. In production, you should display a generic page that says "System down for maintenance" or some other generic message. At no time do you want to tip off a hacker as to what went wrong, nor do you want to display the details of what went wrong. That information is for you alone. Although some developers feel that errors should be displayed during development and even in a production environment - modified, of course, so as not to tip off a hacker - we will assume here that errors will never be displayed. We can set these three parameters in one of two ways. If you have control over the php.ini file then set the following parameters: (See Note 1) error_reporting = E_ALL and Signals and Traps (Oh, My!) - Part 2 by William Shotts, Jr. Errors are not the only way that a script can terminate unexpectedly. You also have to be concerned with signals. Consider the following program: #!/bin/bash echo "this script will endlessly loop until you stop it" while true; do : # Do nothing done After you launch this script it will appear to hang. Actually, like most programs that appear to hang, it is really stuck inside a loop. In this case, it is waiting for the true command to return a non-zero exit status, which it never does. Once started, the script will continue until bash receives a signal that will stop it. You can send such a signal by typing ctrl-c which is the signal called SIGINT (short for SIGnal INTerrupt). Cleaning up after yourself OK, so a signal can come along and make your script terminate. Why does it matter? Well, in many cases it doesn't matter and you can ignore signals, but in some cases it will matter. Let's take a look at another script: #!/bin/bash # Program to print a text file with headers and footers TEMP_FILE=/tmp/printfile.txt pr $1 > $TEMP_FILE echo -n "Print file? [y/n]: " read if [ "$REPLY" = "y" ]; then lpr $TEMP_FILE fi This script processes a text file specified on the command line with the pr command and stores the result in a temporary file. Next, it asks the user if they want to print the file. If the user types "y", then the temporary file is passed to the lpr program for printing (you may substitute less for lpr if you don't actually have a printer attached to your system.) Now, I admit this script has a lot of design problems. While it needs a file name passed on the command line, it doesn't check that it got one, and it doesn't check that the file actually exists. But the problem I want to focus on here is the fact that when the script terminates, it leaves behind the temporary file. Good practice would dictate that we delete the temporary file $TEMP_FILE when the script terminates. This is easily accomplished by adding the following to the end of the script: rm $TEMP_FILE This would seem to solve the problem, but what happens if the user types ctrl-c when the "Print file? [y/n]:" prompt appears? The script will terminate at the read command and the rm command is never executed. Clearly, we need a way to respond to signal
log_errors = On
display_errors = Off
If you don't have control over php.ini, then at the beginning of your script you can set the same parameters using the following: