How to make one field required only if another field is used - php

Question

I have a form where users can enter the names and emails addresses of colleagues. They are not required to enter anything. However if a name is entered then the email address is required.
<form>
1st name <input type="text" name="name_1" value"" />
1st email <input type="text" name="email_1" value"" />
2nd name <input type="text" name="name_2" value""/>
2nd email <input type="text" name="email_2" value""/>
</form>
I can already make certain fields required using the function below. I think I could use an if else statement to check if the 1st name had a value then make the 1st email required. However the form has twenty potential name / email pairs.
What i'm after is advice about the sort of thing I should be trying to do this rather than a complete solution. I appriciate this a bit of a vague question but I'm very new to PHP and am having difficulty searching for a solution.
function check_required_fields($required_array) {
$field_errors = array();
foreach($required_array as $fieldname) {
if (!isset($_POST[$fieldname]) || (empty($_POST[$fieldname]) && !is_numeric($_POST[$fieldname]))) {
$field_errors[] = $fieldname;
}
}
return $field_errors;
}

Answer 1

As long as you are consistent with naming your fields like name_1, you can extract the number from the array keys with substring operations after _ and look for a comparable value in email_.
foreach ($_POST as $key => $value) {
// If it is a name field and isn't empty
if (strpos($key, 'name_') === 0 && !empty($_POST[$key])) {
// The email field is email_ and the number (which comes after the 5th character in name_num)
$namenumber = substr($key, 5);
$emailfield = 'email_' . $namenumber;
// Append the email requirement onto $field_errors if it's empty in $_POST
if (empty($_POST[$emailfield])) {
$field_errors[] = $emailfield;
}
}
}