I have the data in the array form as:

0 => a,
1 => b,
2 => c,
3 => d,
4 => e,
5 => f,
6 => g

How can I convert these array into the following one?

0 => array(0 => a, 1 => b),
1 => array(0 => c, 1 => d),
2 => array(0 => e, 1 => f),
3 => array(0 => g, 1 => null)

Live DEMO

Consider this code:

$j = 0;
for($i=0; $i< count($arr); $i++){
    if(($i !=0) && (($i % 2) == 0)){
        $j++;
    }
    $newArray[$j][] = $arr[$i];
}

$lastElement = count($newArray) -1;
if(count($newArray[$lastElement]) < 2){
    $newArray[$lastElement][]= null;
}
    
echo "<pre>";
print_r($newArray); 

Output:

Array
(
    [0] => Array
        (
            [0] => a
            [1] => b
        )

    [1] => Array
        (
            [0] => c
            [1] => d
        )

    [2] => Array
        (
            [0] => e
            [1] => f
        )

    [3] => Array
        (
            [0] => g
            [1] => 
        )

)