PHP pre-defines several constants and provides a mechanism to define them yourself at runtime. Constants and variables are basically the same. The difference is that constants must be defined using the DEFINE function. Once a constant is defined, it cannot be redefined.
Predefined constants in PHP:
__FILE__
The name of the script file currently being processed. If used in an included file, its value is the included file, not the containing file name.
__LINE__
The current line number of the file being processed.
PHP_VERSION
indicates the current version of the PHP processor, such as: 3.0.8-dev.
PHP_OS
The name of the operating system where the PHP processor is located, such as: Linux.
TRUE
True value
FALSE
False value
You can use the DEFINE function to define more constants.
For example, define constants:
＜?php
define("CONSTANT", "Hello world.");
echo CONSTANT; // outputs "Hello world."
?>
Examples using __FILE__ and __LINE__
＜?php
function report_error(＄file, ＄line, ＄message) {
echo "An error occurred in ＄file on line ＄line: ＄message.";
}
report_error(__FILE__,__LINE__, "Something went wrong!");
?>