Debugging
Workshop Requirements
Before beginning this workshop ensure that you have the ability to upload PHP files to a PHP enabled webserver that has a MySQL database server also. See Setting Up a Server for details on how to install one if you don't already have access.
You should also have a rudimentary understanding of PHP and MySQL. If you do not then you can complete the following Virtual Workshops:
- PHP Web Databases - Part 1
- PHP Web Databases - Part 2
- PHP Web Databases - Part 3
- PHP Web Databases - Part 4
- PHP Web Databases - Part 5
- PHP Web Databases - Part 6
- The MySQL Series of Workshops
You should also have some experience with HTML programming.
Introduction
In the other workshops in this series so far we have concentrated on producing functions to achieve certain results. This workshop is slightly different, more of a reference for what to do when things go wrong (and they will!). This is aimed at novice programmers, as those with experience in other languages will know the principles involved already.
Getting Started
To begin, let's create PHP file with the following code.
<html>
<head><title>Debug Example</title></head>
<body>
<?
echo "<h2>Debugging Workshop</h2>";
echo "<p>We are practicing how to fix various errors</p>";
?>
</body>
</html>
First test that the script works, and next we are going to deliberately break things.
Errors of Omission
PHP is quite good at giving error messages, but for new developers the fact that an error has occurred in the first instance can be quite daunting.
Missing Semi-Colon
The PHP parser requires that we use a semi-colon at the end of each PHP statement, otherwise it gets confused. To look at this type of error remove the semicolon from the first line of PHP code.
echo "<h2>Debugging Workshop</h2>"
echo "<p>We are practicing how to fix various errors</p>";
Which will result in the following error.
Parse error: parse error, unexpected T_ECHO, expecting ',' or ';' in /path/to/debug.php on line 6
Which does actually tell us quite a lot:
1) There has been a parse error: The PHP interpreter has come across an error and doesn't know what it is supposed to do, so just stops.
2) It wasn't expecting another echo (T_ECHO) statement, as it hadn't finished the first one. If the next line after the missing semi-colon was an if statement or a while statement etc then these two would be unexpected (T_IF and T_WHILE etc).
3) What the interpreter was expecting was a ' , ' or a ' ; ' - which we know is what was missing.
4) Which file this was in i.e. '/path/to/debug.php' - useful if you have included functions from different files.
5) Which line the PHP interpreter finds a semi-colon in - which is actually the next line - but does act as an indicator in the script as to where the problem lies.
We can fix this by replacing the semi-colon again.
Missing half of a pair
As we know, PHP uses a series of paired characters to control what happens in the script.
- "Quotation Marks" - used to encapsulate values
- (Brackets) - used to signify a comparison to be made
- {Braces} or {Curly Brackets}- used to contain code to be executed as part of a conditional statement or function.
- The '<%' and '%>' tags that open and close parts of the file that are to be considered PHP code.
Let's have a look at each of these errors in turn.
Missing "Quotation Marks"
echo "<h2>Debugging Workshop</h2>;
echo "<p>We are practicing how to fix various errors</p>";
Parse error: parse error, unexpected '>' in /path/to/debug.php on line 6
This time the parse error occurs because the " is closed after the second echo statement and the PHP interpreter encounters non PHP code (HTML) that it doesn't know what to do with.
Missing (Brackets)
If we add an if condition to test for the presence of the 'Submit' variable passed to the scripts (as in part 4 - Introducing Functions) we can omit the closing bracket of the condition and see the error that generates.
echo "<h2>Debugging Workshop</h2>";
if (!$_REQUEST['Submit'] {
echo "<p>We are practicing how to fix various errors</p";
}
Parse error: parse error, unexpected T_ECHO in /path/to/debug.php on line 7
As we can see this is a similar error: the interpreter finds the echo() function before the condition has been ended by a bracket and knows there has been an error.
Missing {Braces}
This is slightly trickier as there can be lots of nested functions and conditions, which can lead to the problem being more difficult to track down. Let's add a while loop to illustrate (test the output just to make sure that it works).
<?
echo "<h2>Debugging Workshop</h2>";
$count = 1;
while ($count <= 5) {
if (!$_REQUEST['Submit']) {
echo "<p>We are practicing how to fix various errors</p>";
}
$count += 1;
}
We can remove the closing Brace from the if statement:
echo "<h2>Debugging Workshop</h2>";
$count = 1;
while ($count <= 5) {
if (!$_REQUEST['Submit']) {
echo "<p>We are practicing how to fix various errors</p>";
$count += 1;
}
Or the while statement:
echo "<h2>Debugging Workshop</h2>";
$count = 1;
while ($count <= 5) {
if (!$_REQUEST['Submit']) {
echo "<p>We are practicing how to fix various errors</p>";
}
$count += 1;
We end up with the same error message.
Parse error: parse error, unexpected $ in /path/to/debug.php on line 15
This can obviously make debugging a bit more difficult. There are several solutions to overcome this problem.
- Preventative
- Use an editor that highlights the matching pair when one or other is selected (like PHP Edit) allowing you to see mismatched pairs.
- Put a comment after each closing brace to indicate which condition is closed (I find this rather tedious).
- Reactive
- Comment out the nested conditions until the code works as intended then uncomment pairs until you discover the problem.
- You can print out the code and score off matching pairs until you again discover the problem
Missing '?>' tags
Delete the closing PHP tag:
<?
echo "<h2>Debugging Workshop</h2>";
$count = 1;
while ($count <= 5) {
if (!$_REQUEST['Submit']) {
echo "<p>We are practicing how to fix various errors</p>";
}
$count += 1;
}
And we see this error:
Parse error: parse error, unexpected '/' in /path/to/debug.php on line 14
Which is simliar to omitting a closing quotation mark as the PHP interpreter isn't expecting the pure HTML and doesn't know what to do with it.
Problems with Variables
Most problems with variables come from them being misspelled, but there is no error warning to tell us what is wrong when this happens. What usually happens is that 'nothing' happens when we expected something. Consider this example:
<body>
<?
echo "<h2>Debugging Workshop</h2>";
$Message = "<p>We are practicing how to fix various errors</p>";
echo $message;
?>
</body>
The text "We are practicing how to fix various errors" will never be displayed as we have assigned the text to a variable called $Message (with an uppercase M) but echoed the variable $message (lowercase m) which is empty. It is important to ensure that the variable you assign a value to is exactly the same when used later in your script.
Form Problems
Similar problems can happen when using HTML forms and PHP built in super arrays $_GET, $_POST or $_REQUEST. A form field named message could be named with a lower case m.
<input name="message" type="text" />
But called within the script using an uppercase M.
echo $_REQUEST['Message'};
Which would fail also.
'=' Vs '=='
A common use for a variable is as a control for a matching condition and depending if the condition is met or not execute different code (such as we used in earlier workshops for the value of the submit button). To explain let's put the following into our debug.php file.
<body>
<?
echo "<h2>Debugging Workshop</h2>";
$chkvar = "true";
if ($chkvar = "false") {
echo"<p>We are practicing how to fix various errors</p>";
}
?>
</body>
If we run this script the message will be displayed even though the condition has not been met ($chkvar being false). This is because we have not checked the value of $chkvar, simply successfully reassigned the value of the variable. This is why the if condition returns a positive result and displays the message. If we change the operator in the if condition to the correct comparison operator (==) rather than the assignment operator (=) the script will fail to display the message.
<body>
<?
echo "<h2>Debugging Workshop</h2>";
$chkvar = "true";
if ($chkvar == "false") {
echo"<p>We are practicing how to fix various errors</p>";
}
?>
</body>
We can double check this by changing the value of the variable to 'false'.
$chkvar = "false";
Using debug code
In complicated scripts it can be difficult to see where a problem is occurring and thus it is a good idea to include debug code. This may be to see the values of variables used in conditions or even just testing that a control loop has been executed. Place the following into the debug.php file.
<body>
<?
echo "<h2>Debugging Workshop</h2>";
$chkvar = "true";
while ($chkvar == "false") {
if ($count < 5) {
echo "<p>We are practicing how to fix various errors</p>";
} else {
$chkvar = "true";
}
$Count += 1;
}
?>
</body>
This will fail to print the text "We are practicing how to fix various errors". When attempting to correct this we notice that there are two nested conditions which must be met and thus we can introduce debug code to test which one (or both) is failing. To test if the while condition is being met we can echo any text we like between the while and the if statements: I tend to use my name.
$chkvar = "true";
while ($chkvar == "false") {
echo "keith";
if ($count < 5) {
Testing the debug script will fail to echo 'keith' and thus we know there is a problem with the while condition being met. This is because we have set the starting value of $chkvar to 'true' rather than 'false'. Correct this and the script will indeed echo keith, but (along with the text we want) far to many times. This signifies that there is something wrong with the if statement as well. It appears that the $chkvar variable is never being assigned the value true so that the while loop stops. As this relies on the $count variable reaching 5 there is something not working properly. We can comment out our first piece of debug code and place another that echoes the value of the variable being evaluated - $count:
while ($chkvar == "false") {
//echo "keith";
if ($count < 5) {
echo "<p>We are practicing how to fix various errors</p>";
echo $count;
} else {
$chkvar = "true";
}
$Count += 1;
We can see from our test this time that $count is missing and it can be surmised that it has no value. Thus we know it is likely that there is a mismatch between when the $count variable is tested for and when it being assigned. A look at the code used to increase the value of the count variable sees that we have mistakenly used an uppercase C. Changing this to a lowercase c will make the script work as planned.
MySQL debug code
Although we have comprehensively covered how to use error checking code to give useful error messages back to the user, we have missed out one important piece of debug code that is vital to the developer.... displaying the SQL statement if there is an error with its execution. This hasn't been necessary in other workshops as the SQL statements are correct. If we look at error checking code we've used before:
$sql = "SELECT * FRO table";
$result = @mysql_query($sql, $conn);
if (!$result) {
echo("<p>Error performing query: " . mysql_error() . "</p>");
exit();
}
We can see that if the SQL statement fails (there is no result) then we print the error message. Something like:
Error performing query: You have an error in your SQL syntax near 'FRO table' at line 1
If would also be useful to echo the offending SQL statement.
if (!$result) {
echo("<p>Error performing query: " . mysql_error() . "</p>");
echo $sql;
exit();
}
Which would produce an output of:
Error performing query: You have an error in your SQL syntax near 'FRO table' at line 1
Query: SELECT * FRO table;
And we can see that there has been a 'M' missed off the end of FROM in the statement.
Conclusion
When writing your own scripts you may come across more specific problems than this workshop covers. As usual the PHP manual is the best place to look for help and hopefully this Workshop will at least help you identify where the problem lies.
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Posted By: efer at 10:10:12 on Friday the 3rd of February 2012
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