Best method of targeting values in a dynamic form

I have created a website that displays form inputs based on how much the user enters.For example, if the user enters 3 there are displayed 3 groups of inputs.

<form method="post" action="profile.php?create_fitness&amp;step2#tabs-2">
    <input type="number" name="num_exercises" placeholder="Number of Exercises">
    <input type="submit" name="exercises" value="Create">
</form>

Then based on this input I have created a function that displays these inputs based on how many the user entered.The maximum number of these is 15

function echo_exercise_fields($num_fields, $day) {
for($i = 0; $i <= $num_fields - 1;$i++) {
    $ex_number = $i + 1;
    $form_day = 
        '<input type="text" name="workout_' . $day . "_" . $ex_number . '" id="workout" placeholder="Workout"><br>
        <input type="number" name="sets_' . $day . "_" . $ex_number . '" id="sets" placeholder="sets(4)"><br>
        <input type="text" name="reps_' . $day . "_" . $ex_number . '" id="reps" placeholder="reps(10 or 8,10,12)"><br>
        <input type="text" name="rest_' . $day . "_" . $ex_number . '" id="rest" placeholder="rest(30 - 90)"><br>';
    echo "<p>Exercise " . $ex_number . "<p>";
    echo $form_day;
}
}

All the above code works just fine, but then I need to insert these values into a database.I created a separate database just for these inputs with 7 tables that represent the days of the week.

I wrote this code:

foreach($_POST as $key => $value) {
        $exploded = explode("_",$key);
        $element = $exploded[0];
        $day = $exploded[1];
        $exercise_number = $exploded[2];

        $result = $_POST[$element . "_" . $day . "_" . $exercise_number];
}

But then I realized that this won't work.

So my question is how should I proceed in inserting the values that I get from the user into the database.Should I redesign the database?

Any help on this matter would be very much appreciated.