Compound boolean expressions in C/PHP and variable assignment

Question

Forgive me if the title doesn't exactly match what I'm asking, I'm having a tough time describing the question exactly.

Consider the following PHP code:

#!/usr/bin/php
<?php
class foo{
    function bar()
    {
        return true;
    }
}
if (true && $a = new foo() && $a->bar()) {
    echo "true";
} else {
    echo "false";
}

It gives

Fatal error: Call to a member function bar() on a non-object

for the $a->bar(); expression.

Consider the following C code:

int main(void)
{
    int i = 0;

    if (i += 1 && printf("%d\n", i))
    {
     printf("Done: %d.\n", i);
    }
}

It outputs :

0
Done: 1.

Why is this? Since C and PHP claim to short-circuit these expressions and evaluate left to right, I expect a value set in expressions to the left to be used in expressions to the right. Is there something I'm missing?

Answer 1

Your problem right now is with operator precedence. PHP currently evaluates your PHP statement as such:

if (true && $a = ( new foo() && $a->bar() ) )

In which case $a is not defined by the time you try calling $a->bar().

What you really want is this:

if (true && ( $a = new foo() ) && $a->bar())

Using brackets in complex conditions will prevent those kinds of errors from happening.

EDIT: Proof

if(true && $a = true && false) { }
var_dump($a); // bool(false)

if(true && ( $b = true ) && false) { }
var_dump($b); // bool(true)