PHP mysqli real escape string() Function

<?php
$con=mysqli_connect("localhost","my_user","my_password","my_db");
 
// Check connection
if (mysqli_connect_errno()) {
  echo "Failed to connect to MySQL: " . mysqli_connect_error();
}
 
// escape variables for security
$firstname = mysqli_real_escape_string($con, $_POST['firstname']);
$lastname = mysqli_real_escape_string($con, $_POST['lastname']);
$age = mysqli_real_escape_string($con, $_POST['age']);
 
$sql="INSERT INTO Persons (FirstName, LastName, Age)
VALUES ('$firstname', '$lastname', '$age')";
 
if (!mysqli_query($con,$sql)) {
  die('Error: ' . mysqli_error($con));
}
echo "1 record added";
 
mysqli_close($con);
?>