PHP's assert statement doesn't behave like most other languages.
assert('return false'); actually evaluates the string and then asserts its result (false).
Instead of comparing the parameter to true, it goes through the extra step of examining the argument, and if it's a string evaluating it, then performing the comparison.
Very strange indeed.
My problem is not in understanding the behaviour, my problem is coming up with a valid reason for this behaviour, esp. since you now have to do the extra mental work of thinking... "does that evaluate to a string?".
The advantages of a string assertion are less overhead when assertion checking is off and messages containing the assertion expression when an assertion fails. This means that if you pass a boolean condition as assertion this condition will not show up as parameter to the assertion function which you may have defined with the assert_options() function, the condition is converted to a string before calling that handler function, and the boolean FALSE is converted as the empty string.