list(), if and short-circuit evaluation

Question

I have the following code snipplet:

$active_from = '31-12-2009';
if(list($day, $month, $year) = explode('-', $active_from) 
    && !checkdate($month, $day, $year)) {
    echo 'test';
}

Why do I get an undefined variable error?

list($day, $month, $year) = explode('-', $active_from) returns true, so list() is evaluated, isn't it? I think, the variables should be defined? What do I oversee?

This does in my opinion the same and throws no error:

$active_from = '31-12-2009';
list($day, $month, $year) = explode('-', $active_from);
if(checkdate($month, $day, $year)) {
    echo 'test';
}

This raises no error:

if((list($day, $month, $year) = explode('-', $active_from)) && checkdate($month, $day, $year)) {

But I really don't understand why :-)

Thanks for explanation

Answer 1

This is an issue of operator precedence, in your case, the && evaluates before the =, leading to the errors you describe.

You can resolve this problem by placing the assignment statement inside of a parentheses.

Explicitly, your code should read

if(  (list($day, $month, $year) = explode('-', $active_from))
     && !checkdate($month, $day, $year)) {

Note that I have changed it from if( $a=$b && $c ) to if( ($a=$b) && $c ). The parentheses force the assignment operator (=) to evaluate before the conjunction (&&), which is what you want.

Answer 2

Read about operator precedence.

if ( list($day, $month, $year) = explode('-', $active_from) && !checkdate($month, $day, $year) ) {

is identical to

if ( list($day, $month, $year) = (explode('-', $active_from) && !checkdate($month, $day, $year)) ) {