It looks like the callback function registered with assert_options() only gets the third argument set when assert() is called with a string.  Consider the following script.

<?
	function failedAssertion($file, $line, $expression)
	{
		print("'$file', '$line', '$expression'<br>");
	}

	assert_options(ASSERT_ACTIVE, TRUE);
	assert_options(ASSERT_CALLBACK, "failedAssertion");
	assert("1 == 2");
	assert(1 == 2);
?>

This produces the following.

'/usr/local/apache/htdocs/tricks/assert.php', '9', '1 == 2'
'/usr/local/apache/htdocs/tricks/assert.php', '10', ''

If I turn error reporting on, the warning messages mirror this by leaving out the expression.

Looks like the assert.phpt doesn't test assert() for the old form.  Maybe it needs a line like "assert($a != 0);".