Simple if statement returning unexpected results

[eluser]Lone[/eluser]
Ok so right now I feel like a complete noob asking this but can someone explain why the following happens in an if statement in PHP (the third snippet)... Its probably something simple I am missing but just want to see exactly what it is.

A string is treated as 'true' which is expected

Code:

$test1 = true;
$test2 = 'string';
if ($test1 == $test2) {
    echo 'yes';
}
// Outputs 'yes' as expected

The number 0 is treated as 'false' which is expected

Code:

$test1 = false;
$test2 = 0;
if ($test1 == $test2) {
    echo 'yes';
}
// Outputs 'yes' as expected

So if 0 is meant to be 'false' and a string is 'true' then why does the below statement get treated as true?

Code:

$test1 = 0;
$test2 = 'string';
if ($test1 == $test2) {
    echo 'yes';
}
// Outputs 'yes' - NOT expected