Let’s execute following code:

<?php
    $a = array();
    $a[0] = 1;
    $a[1] = null;
    $a[3] = 1;

    for ($i = 0; $i < 4; $i++) {
        echo("index ".$i.":\n");
        echo(" isset: ");
        var_dump(isset($a[$i]));
        echo(" is_null: ");
        var_dump(is_null($a[$i]));
        echo("\n");
    }
?>

The result:

index 0:
 isset: bool(true)
 is_null: bool(false)

index 1:
 isset: bool(false)
 is_null: bool(true)

index 2:
 isset: bool(false)
 is_null: bool(true)

index 3:
 isset: bool(true)
 is_null: bool(false) Notice, that for `$a[2]` as well as for `$a[1]` `is_null` and `isset` return true, so you can't distinguish between set variable with null value from completely unset variable. However you can use `array_key_exists` to do so:

<?php
    $a = array();
    $a[0] = 1;
    $a[1] = null;
    $a[3] = 1;

    for ($i = 0; $i < 4; $i++) {
        echo("index ".$i.":\n");
        echo(" isset: ");
        var_dump(isset($a[$i]));
        echo(" is_null: ");
        var_dump(is_null($a[$i]));
        echo(" array_key_exists: ");
        var_dump(array_key_exists($i,$a));
        echo("\n");
    }
?>

This results with:

index 0:
 isset: bool(true)
 is_null: bool(false)
 array_key_exists: bool(true)

index 1:
 isset: bool(false)
 is_null: bool(true)
 array_key_exists: bool(true)

index 2:
 isset: bool(false)
 is_null: bool(true)
 array_key_exists: bool(false)

index 3:
 isset: bool(true)
 is_null: bool(false)
 array_key_exists: bool(true)

Hope this will help you. It took me a while to figure out.