Stuck on a PHP If Statement

Struggling to code what should be a fairly simple if/else-type statement in PHP. We have a lengthy web form that collects a sport's teams roster (with name, grade and date of birth for each youth player) and then sends that data back to the league's commissioner to be archived for future reference.

I set this form up more than a year ago and it's been working just fine. However, I'd like to tweak it a bit to verify that all of the fields have been completed before executing the script (in case of user error). Here is the code I'm trying to make work:

//Validate player 1:
if (!empty($_POST['player1'])) {
    $player1 = $_POST['player1'];
    $player1_dob = $_POST['player1_dob'];
    $player1_grade = $_POST['player1_grade'];
} elseif (!empty($_POST['player1_dob'])) {
    echo "You forgot to enter Player 1's date of birth.<br>";
    $validate = 'false';
} elseif (!empty($_POST['player1_grade'])) {
    echo "You forgot to enter Player 1's grade.<br>";
    $validate = 'false';
} else {
    $validate = 'true';
}

This seems like the most logical way (to me) to code the statement. However, the script is not verifying the date of birth and grade as intended. Clearly, I am missing an important concept and not using the if/elseif correctly. Perhaps a kind user can help point me in the right direction?