Constants in PHP

PHP defines several constants in advance and provides a mechanism to define them at run time. Constants and variables are basically the same, and the difference is that constants must be defined with the Define function, and once a constant is defined, it cannot be redefined.

Pre-defined constants in PHP:

__file__
This default constant is the name of the PHP program file. If a reference file (include or require) is in the reference file, the constant is the reference file name, not the file name.

__line__
This default constant is the number of PHP program lines. If a reference file (include or require) is in the reference file, the constant is the row that refers to the file, not the file line that references it.

Php_version
This built-in constant is a version of the PHP program, such as ' 3.0.8-dev '.

Php_os
This built-in constant refers to the operating system name, such as ' Linux ', that executes the PHP parser.

TRUE
This constant is the truth (true).

FALSE
This constant is a pseudo value (false).

E_error
This constant refers to the nearest error.

E_warning
This constant refers to the nearest warning.

E_parse
This literal is a potential problem for parsing syntax.

E_notice
This constant is unusual but not necessarily wrong. For example, to access a variable that does not exist.

These e_ are the beginning of the constants, you can refer to the error_reporting () function, there are more relevant instructions.


You can define more constants with the Define function.

For example, define constants:

<?php
Define ("CONSTANT", "Hello world.");
Echo CONSTANT; Outputs "Hello world."
?>

With examples of __file__ and __line__

Php:
function Report_error ($file, $line, $message) {
echo "An error occured in $file to line $line: $message."
}

Report_error (__file__,__line__, "something went wrong!");
?>


My own wording:
?
$file = __file__;
$line = __line__;
Echo $file;
echo "<br><br>";
Echo $line;
echo "<br><br>";
Echo __file__;
echo "<br><br>";
Echo (__line__);
?>

ECHO constant with Echo (); Without echo ""