Everyone loves to dump on Javascript because of stupid things like [] == ![]; // -> true. Some languages are much better at this. But PHP is not among them. I found the following not quite what I would have expected:

boolval("1");   // returns true
boolval("0");   // returns false
boolval(1);     // returns true
boolval(0);     // returns false
boolval("00");  // returns true
boolval("0.0"); // returns true
boolval(0.0);   // returns false