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English
A constant is a value in PHP that cannot be modified after definition. The value of a constant is fixed and is usually used to store some configuration or information that does not require changes. In PHP, there are two types of constants:
Predefined constants : These constants are automatically defined by PHP and do not require a programmer to create manually. For example, PHP_VERSION , PHP_OS , E_ALL , etc.
User-defined constants : These constants are manually defined by the developer through the define() function or the const keyword.
The get_defined_constants() function is used to return all defined constants in the current PHP script. The return value is an associative array, the key of the array is the name of the constant, and the value is the corresponding value of the constant. You can use this function to find constants, especially predefined constants, when debugging PHP code, which is very helpful for debugging PHP environment configuration issues.
To use the get_defined_constants() function, just call it:
<?php
// Get all defined constants
$constants = get_defined_constants();
// Print all constants
print_r($constants);
?>
After running this code, you will get an array output containing all the defined constants. Each element in the array is a constant name and its value. For example, the output might look like:
Array
(
[PHP_VERSION] => 7.4.3
[PHP_OS] => Linux
[E_ALL] => 32767
...
)
There are many predefined constants in PHP, and the get_defined_constants() function returns these constants. Predefined constants are useful for debugging, especially when solving PHP version issues or adjusting the error reporting level. By using this function, you can quickly view all predefined constants that have been defined in the current PHP environment.
For example, you can look for a PHP_VERSION constant that contains information about the current PHP version:
<?php
$constants = get_defined_constants();
echo 'current PHP Version: ' . $constants['PHP_VERSION'];
?>
Sometimes you may be interested only in certain predefined constants. You can display only predefined constants by setting filter conditions. Although the get_defined_constants() function itself does not have a built-in filtering mechanism, you can combine the array_filter() function to filter out constants containing the "PHP_" prefix, which usually represents a predefined constant for PHP:
<?php
$constants = get_defined_constants();
// Filter out all PHP Predefined constants
$php_constants = array_filter($constants, function($key) {
return strpos($key, 'PHP_') === 0;
}, ARRAY_FILTER_USE_KEY);
// Print all PHP Predefined constants
print_r($php_constants);
?>
Through this method, you can view all constants related to the PHP environment, such as PHP version, operating system, etc.
When debugging, you may need to get some predefined constants to help diagnose the problem. For example, you can use get_defined_constants() to get PHP error reporting level constants such as E_ALL , E_NOTICE , etc. If your program has problems handling errors, you can set the error reporting level via the E_ALL constant:
<?php
$constants = get_defined_constants();
error_reporting($constants['E_ALL']);
?>
This way, you can make sure that the error report shows all levels of errors, helping you find the problem in your code.
The get_defined_constants() function returns all constants defined in the current script. If you need to view the value of a specific constant, you can access it directly by the constant name without calling this function.
If you use some custom constants in your code, get_defined_constants() will also list them. So you can check if there are some constants that are not defined as expected.
The get_defined_constants() function is a powerful tool that helps developers view all defined constants during debugging. Whether it is a predefined constant or a user-defined constant, it can be viewed through it to help you better understand and debug PHP programs. Especially when it comes to PHP configuration, error handling, and environment variables, it is valuable to know the values of predefined constants.
