<?php 
$postEmail = 'user@university.ac.uk'; // as $_POST["uni_email"]
if (empty ($postEmail)) {
    $emailErr2 = "University email is required";
} else {
    $uemail = $postEmail;
    if ( !filter_var($uemail, FILTER_VALIDATE_EMAIL)) {
    $emailErr3 = "Invalid email format";
     echo $emailErr3;
    }
}

if (substr(trim($postEmail), -6) === '.ac.uk') {
  echo 'email is correct';
} else {
  $emailErr3 = "Invalid university email format";
  echo $emailErr3;
}
?>

email is correct