Introduction:
An aspect of programming that can be difficult to grasp is the ability to fix problems within your own code. The
difficult part is not when you run into a syntax error, but when the program compiles, but does not work exactly
how it should.
Tricks:
One common way of debugging problems is to have some sort of debugging output. In various different laguages, people
insert print statements in parts of their program to see what a variable value is at a given point, or to see if the
program manages to reach that given point.
In PHP there are two print functions that can prove to be useful, and they are:
| echo | Good for outputting a string or quick debug message |
| print_r | Quick way to print a whole array or object |
Example:
Here we will not use simple examples, instead the readers should try debugging the example with the article.
Example 1:
<html>
<body>
<?php
if ($_POST["submit"]=="submit") {
$var1=str_replace(array("<",">"),"",$_POST["var1"]);
echo $_POST["var1"];
}
?>
<form method="post">
<input type="text" name="var1">
<input type="image" name="submit"
src="http://www.theserverpages.com/files/ex-submit.gif"
value="submit" border="0">
</body>
</html>
For those that are too lazy to try this on their own servers, here is a link
Now if you try the above link in FireFox, it works perfectly and ends up printing what is in the text box when you hit submit. If you
tried the same thing in Internet Explorer, you will see that the page only refreshes when you hit submit. So what went wrong here?
If you know PHP well, you can probably skip alot of debugging work and almost get straight to the answer by running the code in your mind. I will pretend like I do not have a clue why this is happening, so the first thing I would do is check wether it is posting, and wether the if statement works.
The lines highlighted in blue are inserted into the code to provide some sort of debugging output.
<html>
<body>
<?php
print_r($_POST); // use of print_r
if ($_POST["submit"]=="submit") {
echo "if statement is working..."; // use of echo
$var1=str_replace(array("<",">"),"",$_POST["var1"]);
echo $_POST["var1"];
}
?>
<form method="post">
<input type="text" name="var1">
<input type="image" name="submit"
src="http://www.theserverpages.com/files/ex-submit.gif"
value="submit" border="0">
</body>
</html>
Here is the link for this script: link
FireFox output:
Internet Explorer output:
As we see from the outputs, there is not $_POST["submit"] variable when using Internet Explorer. So we should check if $_POST["submit_x"] and $_POST["submit_y"] are set.
<html>
<body>
<?php
if (isset($_POST["submit_x"]) && isset($_POST["submit_y"])) {
$var1=str_replace(array("<",">"),"",$_POST["var1"]);
echo $_POST["var1"];
}
?>
<form method="post">
<input type="text" name="var1">
<input type="image" name="submit"
src="http://www.theserverpages.com/files/ex-submit.gif"
value="submit" border="0">
</body>
</html>
Here is the link for the final code: link
