Debugging with PHP

In this article, we look at various ways of debugging PHP scripts, from printing variable values to using sophisticated debuggers.

Reporting Errors

PHP generates various error messages according to it’s configuration in `php.ini` but these are suppressed by default, since it isn’t logical for a user to see them. For debugging purposes however, they can be switched on.
The values of the following variables in `php.ini` need to be set appropriately :

`display_errors` to 1
`error_reporting` to desired level of error reporting (eg: E_ALL & ~E_NOTICE)

On shared hosts, it might not be possible to alter `php.ini`. However, the same changes can be conveyed through the `.htaccess` file :

php_flag display_errors on
php_value error_reporting 2039

The value of error_reporting is a bit-wise combination of the various kinds of errors that are possible to report. Check out this link for more info.
The simplest way to enable reporting however, maybe to add equivalent lines to the running script.

ini_set('display_errors', 1);
error_reporting(-1); // report all errors

A slight modification can be made above to ensure that `display_errors` is only set on local/dev machine.

$testServer = $_SERVER['SERVER_NAME'] = "127.0.0.1" || $_SERVER['SERVER_NAME'] = "localhost";
ini_set('display_errors', $testServer);

It needs to be mentioned that the parameter `display_errors` need not be set in order to view errors. If `error_reporting` is set, then the errors are logged in the PHP log, and can be read from there. The `error_log` entry in `php.ini` declares the location of the log file.

Just printing the variables

An elementary way to try to debug code is to print the relevant variables and possibly, have the script exit at those points. This can be done in a couple of ways.

`print_r`

echo '<pre>';
print_r($foo);
print_r($bar);
echo '</pre>';

`var_dump`

var_dump($foo, $bar);
// exit();

The output of `print_r` and `var_dump` differs slightly in the formatting and variable information printed. Also, `print_r` doesn’t take multiple arguments.
While this might seem like a trivial way to debug applications, but using it at appropriate points found by analyzing the application structure can be quite useful.

Backtrace

The function `debug_print_backtrace` prints the PHP backtrace when called. While it can be put anywhere, it can be particularly useful in an exception handler.

set_exception_handler('exception_handler');

function exception_handler(Exception $e){
// do things
// might dump some variables here for added info
debug_print_backtrace();
exit();
}

Registering a custom Error Handler

An error handler is called whenever an error occurs. The advantage of using an error handler is that no matter what the value of `error_reporting` is, the handler is always called and errors can be logged.

function logErrors($errorNum, $errorStr){
file_put_contents("/tmp/php_errors.php", date('Y-m-d H:i:s - ') . $errorStr, FILE_APPEND);
}
set_error_handler('logErrors');

Using Debuggers

A number of tools can be used as editors and/or debuggers with PHP and popular editors like NetBeans/Eclipse. Some of these tools are mentioned below :