Example is <https://3v4l.org/33O0B>:

<?php

var_dump(file_get_contents("\0"));

File path cannot contain NUL char.

	/* Parse arguments */
	if (zend_parse_parameters(ZEND_NUM_ARGS(), "p|br!ll", &filename, &filename_len, &use_include_path, &zcontext, &offset, &maxlen) == FAILURE) {
		return;
	}

"p" is used and "\0" results in invalid path and zend_parse_parameters() fails. That's the reason why it returns NULL.

All other functions that expects file path returns NULL for "\0" due to zend_parse_parameters() failure.

Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

See the note on <http://php.net/manual/en/functions.internal.php>:

> Note: If the parameters given to a function are not what it
> expects, such as passing an array where a string is expected,
> the return value of the function is undefined. In this case it
> will likely return NULL but this is just a convention, and
> cannot be relied upon.