These constants are defined by the PHP core.

PHP_EOL The correct ‘End Of Line’ symbol for this platform. Available since PHP 5.0.2
PHP_INT_MAX The largest integer supported in this build of PHP. Usually int(2147483647) in 32 bit systems and int(9223372036854775807) in 64 bit systems. Available since PHP 5.0.5
PHP_INT_MIN The smallest integer supported in this build of PHP. Usually int(-2147483648) in 32 bit systems and int(-9223372036854775808) in 64 bit systems. Available since PHP 7.0.0. Usually, PHP_INT_MIN === ~PHP_INT_MAX.
PHP_INT_SIZE The size of an integer in bytes in this build of PHP. Available since PHP 5.0.5
PHP_FLOAT_EPSILON Smallest representable positive number x, so that x + 1.0 != 1.0. Available as of PHP 7.2.0.
PHP_FLOAT_MIN Smallest representable floating point number. Available as of PHP 7.2.0.
PHP_FLOAT_MAX Largest representable floating point number. Available as of PHP 7.2.0.

Note

It is better to use PHP_EOL when you need to put new line characters. Since the difference between new line characters in different systems, PHP_EOL can improve the portability of your codes to some extent.

References