MySQL & PHP | Update doesnt work after select

I have this weird situation where my query isn't doing what it's supposed to do.

Here is my not working code:

$aanbodID = 1;

$getData = mysql_query("SELECT boekbaar_iframe FROM tbl WHERE id_aanbod= $aanbodID");
while ($row_content = mysql_fetch_array($getData)) 
{       
    $zichtbaarjanee = $row_content['boekbaar_iframe']; // 0 or 1
}

$nieuwewaarde = 0;

if ($zichtbaarjanee == 0)
{
    $nieuwewaarde = 1;
}

mysql_query("UPDATE aanbod_20160206 SET boekbaar_iframe = ".$nieuwewaarde." WHERE id_aanbod = ".$aanbodID."");
echo mysql_error();

And here is almost the same code that is working:

$aanbodID = 1;

$getData = mysql_query("SELECT boekbaar_iframe FROM tbl WHERE id_aanbod= $aanbodID");
while ($row_content = mysql_fetch_array($getData)) 
{       
    $zichtbaarjanee = $row_content['boekbaar_iframe']; // 0 or 1
}

//switch these vars and its working    
$nieuwewaarde = 0;
// $nieuwewaarde = 1; 

mysql_query("UPDATE tbl SET boekbaar_iframe = ".$nieuwewaarde." WHERE id_aanbod = ".$aanbodID."");
echo mysql_error();

So i'm guessing the problem is somewhere in the if statement, but i've tried everything, also with and without ' ' or " ".

Please help!