Here's a simple PHP script that connects to a MySQL database and performs CRUD operations on the "employees" table. Please make sure to replace the placeholders with your actual database credentials.

```php
<?php

// Database connection
$servername = "your_servername";
$username = "your_username";
$password = "your_password";
$dbname = "your_dbname";

$conn = new mysqli($servername, $username, $password, $dbname);

if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

// Function to sanitize user input
function clean_input($data) {
  $data = trim($data);
  $data = stripslashes($data);
  $data = htmlspecialchars($data);
  return $data;
}

// Create employee
function create_employee($first_name, $last_name, $email, $hire_date) {
    global $conn;

    // Validate user input
    if (empty($first_name) || empty($last_name) || empty($email) || empty($hire_date)) {
        echo "All fields are required.";
        return;
    }

    if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
        echo "Invalid email format.";
        return;
    }

    $sql = "INSERT INTO employees (first_name, last_name, email, hire_date)
            VALUES ('$first_name', '$last_name', '$email', '$hire_date')";

    if ($conn->query($sql) === TRUE) {
        echo "New employee created successfully.";
    } else {
        echo "Error: " . $sql . "<br>" . $conn->error;
    }
}

// Read all employees
function read_employees() {
    global $conn;

    $sql = "SELECT id, first_name, last_name, email, hire_date FROM employees";
    $result = $conn->query($sql);

    if ($result->num_rows > 0) {
        while($row = $result->fetch_assoc()) {
            echo "id: " . $row["id"]. " - Name: " . $row["first_name"]. " " . $row["last_name"]. " - Email: " . $row["email"]. " - Hire Date: " . $row["hire_date"]. "<br>";
        }
    } else {
        echo "No employees found.";
    }
}

// Update employee
function update_employee($id, $first_name, $last_name, $email, $hire_date) {
    global $conn;

    // Validate user input
    if (empty($id) || empty($first_name) || empty($last_name) || empty($email) || empty($hire_date)) {
        echo "All fields are required.";
        return;
    }

    if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
        echo "Invalid email format.";
        return;
    }

    $sql = "UPDATE employees SET first_name='$first_name', last_name='$last_name', email='$email', hire_date='$hire_date' WHERE id=$id";

    if ($conn->query($sql) === TRUE) {
        echo "Employee updated successfully.";
    } else {
        echo "Error updating employee: " . $conn->error;
    }
}

// Delete employee
function delete_employee($id) {
    global $conn;

    // Validate user input
    if (empty($id)) {
        echo "ID is required.";
        return;
    }

    $sql = "DELETE FROM employees WHERE id=$id";

    if ($conn->query($sql) === TRUE) {
        echo "Employee deleted successfully.";
    } else {
        echo "Error deleting employee: " . $conn->error;
    }
}

// Example usage
$first_name = clean_input($_POST["first_name"]);
$last_name = clean_input($_POST["last_name"]);
$email = clean_input($_POST["email"]);
$hire_date = clean_input($_POST["hire_date"]);

create_employee($first_name, $last_name, $email, $hire_date);
read_employees();
update_employee(1, $first_name, $last_name, $email, $hire_date);
delete_employee(2);

$conn->close();

?>
```

This script defines functions for creating, reading, updating, and deleting employees in the "employees" table. It also includes a function to sanitize user input and validate email addresses. You can call these functions with appropriate parameters to perform CRUD operations.

Please note that this example is not using prepared statements or any advanced security measures, so it's not recommended for production use without further improvements.
다음은 MySQL 데이터베이스에 연결하여 "employees" 테이블에서 CRUD 작업을 수행하는 간단한 PHP 스크립트입니다. 플레이스홀더를 실제 데이터베이스 자격 증명으로 바꾸어야 합니다.

```php
<?php

// Database connection
$servername = "your_servername";
$username = "your_username";
$password = "your_password";
$dbname = "your_dbname";

$conn = new mysqli($servername, $username, $password, $dbname);

if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

// Function to sanitize user input
function clean_input($data) {
  $data = trim($data);
  $data = stripslashes($data);
  $data = htmlspecialchars($data);
  return $data;
}

// Create employee
function create_employee($first_name, $last_name, $email, $hire_date) {
    global $conn;

    // Validate user input
    if (empty($first_name) || empty($last_name) || empty($email) || empty($hire_date)) {
        echo "All fields are required.";
        return;
    }

    if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
        echo "Invalid email format.";
        return;
    }

    $sql = "INSERT INTO employees (first_name, last_name, email, hire_date)
            VALUES ('$first_name', '$last_name', '$email', '$hire_date')";

    if ($conn->query($sql) === TRUE) {
        echo "New employee created successfully.";
    } else {
        echo "Error: " . $sql . "<br>" . $conn->error;
    }
}

// Read all employees
function read_employees() {
    global $conn;

    $sql = "SELECT id, first_name, last_name, email, hire_date FROM employees";
    $result = $conn->query($sql);

    if ($result->num_rows > 0) {
        while($row = $result->fetch_assoc()) {
            echo "id: " . $row["id"]. " - Name: " . $row["first_name"]. " " . $row["last_name"]. " - Email: " . $row["email"]. " - Hire Date: " . $row["hire_date"]. "<br>";
        }
    } else {
        echo "No employees found.";
    }
}

// Update employee
function update_employee($id, $first_name, $last_name, $email, $hire_date) {
    global $conn;

    // Validate user input
    if (empty($id) || empty($first_name) || empty($last_name) || empty($email) || empty($hire_date)) {
        echo "All fields are required.";
        return;
    }

    if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
        echo "Invalid email format.";
        return;
    }

    $sql = "UPDATE employees SET first_name='$first_name', last_name='$last_name', email='$email', hire_date='$hire_date' WHERE id=$id";

    if ($conn->query($sql) === TRUE) {
        echo "Employee updated successfully.";
    } else {
        echo "Error updating employee: " . $conn->error;
    }
}

// Delete employee
function delete_employee($id) {
    global $conn;

    // Validate user input
    if (empty($id)) {
        echo "ID is required.";
        return;
    }

    $sql = "DELETE FROM employees WHERE id=$id";

    if ($conn->query($sql) === TRUE) {
        echo "Employee deleted successfully.";
    } else {
        echo "Error deleting employee: " . $conn->error;
    }
}

// Example usage
$first_name = clean_input($_POST["first_name"]);
$last_name = clean_input($_POST["last_name"]);
$email = clean_input($_POST["email"]);
$hire_date = clean_input($_POST["hire_date"]);

create_employee($first_name, $last_name, $email, $hire_date);
read_employees();
update_employee(1, $first_name, $last_name, $email, $hire_date);
delete_employee(2);

$conn->close();

?>
```

이 스크립트는 "employees" 테이블에서 직원을 만들고, 읽고, 업데이트하고, 삭제하는 함수를 정의합니다. 또한 사용자 입력을 살균하고 이메일 주소의 유효성을 검사하는 함수도 포함되어 있습니다. 적절한 매개 변수를 사용하여 이러한 함수를 호출하여 CRUD 작업을 수행할 수 있습니다.

이 예제에서는 준비된 문이나 고급 보안 조치를 사용하지 않으므로 추가 개선 없이 프로덕션용으로 사용하는 것은 권장하지 않습니다.