function validate_select($val, $myoptions)
{
//print_r($myoptions);
for($i=0;$i<count($myoptions);$i++){
    if($val==$myoptions[$i]){
        return $val;
    }
}
    return false;
}

$testVar = 'apple';
$myoptions = array('banana','pear','apple');
$result = filter_var($testVar, FILTER_CALLBACK, array('options' => function($var) {
  //return validate_select($var, array('banana','pear','apple')); //case A: returns correct value 'apple'
  return validate_select($var, $myoptions); //case B: returns unexpected value false
}));
echo($result);

$result = filter_var($testVar, FILTER_CALLBACK, array('options' => function ($var) use ($myoptions) {
    return validate_select($var, $myoptions);
}));