I'm try to add an array element to an array (if a certain condition is met) before I push that row into a result array.

Right now, it adds the value as a separate, single-element row.

for ($i = 0; $i < 4; $i++) {
    $ret1[] = array("A" . $i, "B" . $i);
    if ($i > 2) {
        $ret1[] = array("C" . $i);
    }
}    
print_r($ret1);

Current result:

Array
(
    [0] => Array
        (
            [0] => A0
            [1] => B0
        )

    [1] => Array
        (
            [0] => A1
            [1] => B1
        )

    [2] => Array
        (
            [0] => A2
            [1] => B2
        )

    [3] => Array
        (
            [0] => A3
            [1] => B3
        )

    [4] => Array
        (
            [0] => C3
        )
)

Expected result:

(
    [0] => Array
        (
            [0] => A0
            [1] => B0
        )

    [1] => Array
        (
            [0] => A1
            [1] => B1
        )

    [2] => Array
        (
            [0] => A2
            [1] => B2
        )

    [3] => Array
        (
            [0] => A3
            [1] => B3
            [2] => C3
        )
)

You can use a temporary variable to define the array. Then you can decide to push another element to it based on your condition. Finally you can push the temporary array to your $ret1 array to achieve the desired result.

for ($i=0; $i<5; $i++) {
    $arr = ["A$i", "B$i"];
    if ($i > 2) {
        $arr[] = "C$i";
    }
    $ret1[] = $arr;
}    

What you're doing is pushing 2 elements to the array in the last 2 iterations of your loop. One with a value of ["A3", "B3"] and another with a value of ["C3"] which just results in [["A3", "B3"], ["C3"]], which isn't what you're after. By using the temporary variable $arr we defer pushing the final array to $ret1 until after the conditional statement is executed or bypassed.

So for example, in the penultimate iteration of this loop the value of $arr is initially ["A3","B3"], then we push another value "C3" to the end of $arr based on $i > 2 being true, which makes $arr = ["A3","B3","C3"], and finally we push $arr to the end of the array $ret1 giving us the final result [ ..., 3 => ["A3","B3","C3"], ...]

Bonus notes

None of this is critical to your problem or your question, but I figured I'd throw it out there just in case.

In PHP, double quoted strings give you automatic variable expansion. Which means that "A$i" === "A" . $i. So just something to consider. Additionally, there is a short-hand syntax for arrays, which has been available since PHP 5.4.0 and in my personal opinion is easier to write and read than using the traditional array() construct. So array("A" . $i, "B" . $i) === ["A$i", "B$i"].