php数组检查结果没有任何意义

the following code:

<?php
$col_names = array('n1','n2','n3','n4','n5');
$values    = array(0,0,'NOW()',0,0);
$i = 0;
while($i<count($col_names)) {
    if($values[$i] == 'NOW()') {
        $qmarks[] = "NOW()";    
    } else {
        $qmarks[] = '?';    
    }    
    $i++;
}
print_r($qmarks);
?>

results this:

Array
(
    [0] => NOW()
    [1] => NOW()
    [2] => NOW()
    [3] => NOW()
    [4] => NOW()
)

And it should be this:

Array
(
    [0] => ?
    [1] => ?
    [2] => NOW()
    [3] => ?
    [4] => ?
)

Does anybody have an Idea why?

<?php
$col_names = array('n1','n2','n3','n4','n5');
$values    = array(0,0,'NOW()',0,0);
$i = 0;
while($i<count($col_names)) {
    if($values[$i] === 'NOW()') {
        $qmarks[] = "NOW()";    
    } else {
        $qmarks[] = '?';    
    }    
    $i++;
}
print_r($qmarks);
?>

put === instead ==