Php developers often face a set of common errors in different situations; however, the root cause(s) of these errors are ideally the same. It is quite easy to fix these errors once developers understand the problem and are able to locate the wrong or missing code and make the necessary changes.

Here a few such errors and their generic fixes compiled for quick reference. Simply look up the error message from this reference and apply the general fix to the code. If the problem needs further investigation, you can also browse through the links provided to get additional information on how to fix the error.

A List of Common PHP Errors:

• Nothing is seen. The page is empty and white. (also known as White Page or Screen Of Death)

Notices

• Undefined variable
• Undefined Index
• Use of undefined constant xxx – assumed ‘xxx’

Parse Errors

• syntax error, unexpected T_XXX
• syntax error, unexpected T_ENCAPSED_AND_WHITESPACE
• syntax error, unexpected T_PAAMAYIM_NEKUDOTAYIM

Warnings

• Cannot modify header information – headers already sent
• mysql_fetch_array() expects parameter 1 to be resource, boolean given
• [function]: failed to open stream: [reason]
• open_basedir restriction in effect

Fatal Errors

• Allowed memory size of xxxx bytes exhausted (tried to allocate xxx bytes)
• Call to a member function … on a non-object
• Call to Undefined function
• Cannot redeclare xxx
• Can’t use function return value in write context
• Declaration of AAA::BBB() must be compatible with that of CCC::BBB() ‘
• Using $this when not in object context

MySql Errors

• You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near … at line …

Nothing is seen. The page is empty and white.

The White Page or better known as the Screen of Death, this is a common problem faced when error reporting is disabled. Fatal errors especially syntax errors result in a blank page as the system does not display the corresponding error message on screen.

If the system has been configured to log errors, the complete error message though not flashed on screen, will be recorded in the error log.

Error reporting is usually disabled in the deployed version so that website users are not exposed to syntax and parsing errors. Error logging should always be enabled to keep track and fix script errors accurately, as logs hold valuable information on the erroneous line of code, and the error message.

The simple solution to this problem is to temporarily enable error reporting so that error messages are visible on the screen. This may however cause all errors messages to be visible to all users of the website.

Add the following code at the top of the PHP code to enable error reporting:

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ini_set('display_errors', 1); error_reporting(~0);

The ini_set() function will enable the display of errors and set error reporting to the highest level to trap all possible errors in the script. However ini_set() works only at run time and does not display syntax or parsing errors, but they are still written to the error log.

To display such errors in the browser window:

• Set the display_startup_errors directive to true in a .htaccess file or the php.ini (or)
• Specify the required settings using other options that affect the runtime configuration

Notice: Undefined variable

This notice occurs when a variable is used without being defined previously.

For Example:

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foreach ($items as $item) {
    // do something with item
    $counter++;
}

This script will trigger a notice if the variable $counter has not been defined earlier in the script. Strict coding standards require the variable to be defined and set with a default value based on the data type before it is used.

In this case the variable initially gets defined and is set to 0 before being incremented.

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$counter = 0;
foreach ($items as $item) {
    // do something with item
    $counter++;
}

Notice: Undefined Index

This notice is displayed when a script tries to access an array using a key value that is not present in the array.

The following code is a classic example for an Undefined Index notice:

$data = array('foo' => '42', 'bar');
echo $data['spinach'];
echo $data[1];

Both key values, spinach and 1, used to access the array do not exist in $data, causing an E_NOTICE to be raised.

The best way to address this problem is to first check if the offset, index or key value exists in the array before accessing the array element.

Use array_key_exists or isset to check existence of array index.

$data = array('foo' => '42', 'bar');
if (array_key_exists('spinach', $data)) {
echo $data['spinach'];
}
else {
echo 'No key spinach in array';
}

Notice: Use of undefined constant xxx – assumed ‘xxx’

This notice is again associated with vague definition of tokens. Tokens are assumed as constants based on their usage, but there are no explicit definitions.

Common causes that trigger this notice include:

• A string used as an associative array key not enclosed within quotes
  • echo $array[key]; // Wrong
  • echo $array['key']; // Right
• Variable name without a preceding $ (dollar) sign
  • echo varName; // Wrong
  • echo $varName; // Right

Often the notice is also caused when the script tries to access a constant and the library or PHP extension that contains the constant definition is not available in the specified directory.