Bug in function, in_array()??

[Ipsens]

So...
This is user input:

 $range = array(2.55, 59.69); 

This is parsed code:

 $okRange = array('min_range', 'max_range');
 
foreach($range as $k => $v)
{
   if( in_array($k, $okRange) === false )
   {
     echo "$k is invalid range flag!" . NL;
   }
} 

Response is:

1 is invalid range flag!

But it was supposed to fail at: 0 is invalid range flag!
Why did int(0), passed through?!?

After I added a third param bool(true), to in_array(), then it worked like:

... if( in_array($k, $okRange, true) === false ) ... 

Bit it still doesn't make sense, why did int(0), passed through without third param bool(true)?!?

int(0), was compared to values 2.55 and 59.69.
So it was supposed to fail, like int(1) does!