Predefined Constants¶
Predefined constants are PHP native constants, available at the start of the PHP engine.
Those constants are for internal usage, such as E_ALL, for error reporting, or STDOUT, for output. Some constants are providing information, such as PHP_VERSION’s PHP version number. Others are meant to give a readable name to common values or arguments, such as DATE_RFC822.
Other constants come from extensions : for example, the Math extension defines M_PI for the pi constant (3.14159…), libsodium defines SODIUM_CRYPTO_CORE_RISTRETTO255_SCALARBYTES, etc. Those are also predefined constants, although they depend on the compilation of PHP.
<?php
echo "This is PHP ".PHP_VERSION;
?>
See also Understanding PHP Predefined Constants
Related : Constants