php validate post request before add to data base - php

Question

i have an a project im using php and flutter for that
when i want to add a data to database i want to validate user inputs in php
im using a postman to test it i want to all inputs correct that add on database can some one help me?
when i request a post add and user want to add data to database i want all inputs correct as i defined on below if one inputs not correct i want to tell that filed incorrect and do not add a data utill all inputs correct
if ($_SERVER["REQUEST_METHOD"] === 'POST') {
$pattern = "^[_a-z0-9-]+(\.[_a-z0-9-]+)*#[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3})$^";
if (empty($_POST['First_Name'])) {
echo 'Error! You didnt Enter the First Name. <br />';
} else {
$first_name = $_POST['First_Name'];
if (!preg_match("/^[a-zA-z]*$/", $first_name)) {
echo 'Only alphabets and whitespace are allowed For First Name. <br />';
}
}
if (empty($_POST['Last_Name'])) {
echo 'Error! You didnt Enter the Last Name. <br />';
} else {
$last_name = $_POST['Last_Name'];
if (!preg_match("/^[a-zA-z]*$/", $last_name)) {
echo 'Only alphabets and whitespace are allowed For Last Name. <br />';
}
}
if (empty($_POST['Email'])) {
echo 'Email Address Is Required <br />';
} else {
$email = $_POST['Email'];
if (!preg_match($pattern, $email)) {
echo 'Email is not valid! <br />';
}
}
if (empty($_POST['Phone'])) {
echo 'Phone Number is Required! <br />';
} else {
$phone = $_POST['Phone'];
if (!preg_match('/^[0-9]*$/', $phone)) {
echo 'Only Numeric Value Is Allowed. <br />';
} elseif (!preg_match('/^0\d{10}$/', $phone)) {
echo 'Invalid Phone Number!';
} elseif (preg_match('/^0\d{10}$/', $phone)) {
$re = "SELECT * FROM user WHERE Phone=$phone ";
$reresult = mysqli_query($conn, $re);
if (mysqli_num_rows($reresult) > 0) {
echo "user has already registered! <br />";
}
}
}
$first_name = mysqli_real_escape_string($conn, $_POST['First_Name']);
$last_name = mysqli_real_escape_string($conn, $_POST['Last_Name']);
$email = mysqli_real_escape_string($conn, $_POST['Email']);
$phone = mysqli_real_escape_string($conn, $_POST['Phone']);
$dob = mysqli_real_escape_string($conn, $_POST['DOB']);
$sql = "INSERT INTO `user` (`First_Name`,`Last_Name`,`Email`,`Phone`,`DOB` )
VALUES('$first_name','$last_name','$email','$phone','$dob')";
$query = mysqli_query($conn, $sql);
//$check=mysqli_fetch_array($query);
if ($query) {
echo ' user successfully added!';
} else {
echo 'failure';
}
//phone else
}
im asking to solve my problem