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PHP Tutorial: Error Handling

By Vince Barnes

It is the mark of a good programmer that they do not stop once they have found a way to make something work. Instead, the conscientious programmer sets about finding every way that their solution might not work, and then writes some code to cover those situations. Quality program code handles all foreseeable error situations, gracefully informing the user of the error that has occurred, and not just crashing, going away and leaving the user's mouth hanging open having just said "wha'....???"

This type of code is known as Error Handling, and we're going to apply some to the file upload solution we just created. This was our "getfile.php" page:

<html>
<head>
<title>Process Uploaded File</title>
</head>
<body>
<?php

move_uploaded_file ($_FILES['uploadFile'] ['tmp_name'],
"../uploads/{$_FILES['uploadFile'] ['name']}")

?>
</body>
</html>

Now we have to think of the kinds of error situations that might occur. The first, and perhaps the most obvious, is that something goes wrong in the middle of the file transfer, and only a part of the file makes it through. It is also possible that for some reason (perhaps to do with the user's file selection, or permissions or any other such reason) no file was uploaded at all.

There are also some possible errors that could arise because of capabilities in PHP itself. PHP can be configured on the server to only allow file transfers of files up to a certain size. When this size is exceeded, an error occurs. We can also set a file size limit ourselves by adding a MAX_FILE_SIZE limit to our HTML form. To do this, we would add a line like this one inside our <form>:

I'm showing a maximum size of 25,000 bytes here, but you can use any size you wish.

Now to handle those errors! PHP is not going to leave us high and dry. Instead, it includes some very helpful features.

First, we need to know if our file upload was successful or if any error occurred. Happily, we can test the result of the move_uploaded_file function directly. If it was successful, it will return "true", if any error occurred, it will return "false", which enable us to test it with an "if" statement like this:

In this example, I've left out the actual error handling code for the moment so that you can clearly see how the "if" tests the true/false return from the move_uploaded_file function.

Now we need to find out what kind of error occurred and do something about it. You will remember from the last part of this series that we discussed the $_FILES array and is associative indexes. There is anothe such index called 'error' that returns a value to indicate what kind of error has occurred. We can conveniently test this value by using a "switch.... case" construct.

A "switch.... case" construct is like coding a series of "if.... else" statements, except that it is much tidier and easier to read. Basically the "switch" part specifies a variable to be tested for a series of values, and the "case" parts say what to do in "case the value is this". This is what it looks like when it is applied to our example:

switch ($_FILES['uploadFile'] ['error'])
{ case 1:
           print '<p> The file is bigger than this PHP installation allows</p>';
           break;
    case 2:
           print '<p> The file is bigger than this form allows</p>';
           break;
    case 3:
           print '<p> Only part of the file was uploaded</p>';
           break;
    case 4:
           print '<p> No file was uploaded</p>';
           break;
}

We now have code to handle the error situation we discussed earlier! Putting it all together, we get the following code (showing first the form, then the PHP page that handles the form):

<html>
<head>
<title>File Upload Form</title>
</head>
<body>
This form allows you to upload a file to the server.<br>

<form action="getfile.php" method="post"><br>
Type (or select) Filename: <input type="file" name="uploadFile">
<input type="hidden" name="MAX_FILE_SIZE" value="25000" />
<input type="submit" value="Upload File">
</form>

</body>
</html>

and now the PHP page to handle the form:

<html>
<head>
<title>Process Uploaded File</title>
</head>
<body>
<?php

if ( move_uploaded_file ($_FILES['uploadFile'] ['tmp_name'],
       "../uploads/{$_FILES['uploadFile'] ['name']}") )
      { print '<p> The file has been successfully uploaded </p>';
       }
else
      {
        switch ($_FILES['uploadFile'] ['error'])
         { case 1:
                   print '<p> The file is bigger than this PHP installation allows</p>';
                   break;
            case 2:
                   print '<p> The file is bigger than this form allows</p>';
                   break;
            case 3:
                   print '<p> Only part of the file was uploaded</p>';
                   break;
            case 4:
                   print '<p> No file was uploaded</p>';
                   break;
         }
       }

?>
</body>
</html>

We now have a much more elegant file upload process!

Brainware IT Sanchar

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Thursday, April 11, 2013

PHP Tutorial: Error Handling

PHP Tutorial: Error Handling

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