Constant is a name of simple value (number, text), which should not change during code execution. Constants are case sensitive, and good programming style requires use them in uppercase.
define("FOO", "something");
During script execution large number of constants is available defined by php engine. For example, M_PI (3.1415926535898).
Exception is magic constants which value depends on where they are requested [2].
Let us have a look at an example:
<?php
$s = new Sample;
$s->test(123);
class Sample {
function test($arg) {
echo __LINE__ . "<br>";
echo __FILE__ . "<br>";
echo __DIR__ . "<br>";
echo __FUNCTION__ . "<br>";
echo __CLASS__ . "<br>";
echo __METHOD__ . "<br>";
echo __NAMESPACE__ . "<br>";
}
}
|
Constant |
Description |
Value |
|
PHP_VERSION |
|
5.2.6 |
|
__LINE__ |
he current line number of the file. |
9 |
|
__FILE__ |
The full path and filename of the file. |
/var/www/sample.php |
|
__DIR__ |
The directory of the file. Equivalent dirname(__FILE__). Available since 5.3.0 |
__DIR__ |
|
__FUNCTION__ |
This constant returns the function name as it was declared. |
test |
|
__CLASS__ |
This constant returns the class name as it was declared. |
Sample |
|
__METHOD__ |
The method name is returned as it was declared. Available since 5.0.0 |
Sample::test |
|
__NAMESPACE__ |
The name of the current namespace. Available since 5.3.0 |
__NAMESPACE__ |
By the way __LINE__ is very useful detecting sql queries returning error. Example:
$sql = 'SELECT * FROM nothing';
$result = mysql_query($sql) or die(mysql_error() . ' /
#' . __LINE__ . ', ' . __FILE__ . '<pre>' . $sql . '</pre>');
Result:
No database selected / #15, /var/www/sample.php
SELECT * FROM nothing
Sources used:
1. http://www.php.net/manual/en/reserved.constants.php
2. http://www.php.net/manual/en/language.constants.predefined.php
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