Ans :-
There is no guarantee that the information provided by the user is always correct. PHP validates the data at the server-side, which is submitted by HTML form. You need to validate a few things:

Empty String
Validate String
Validate Numbers
Validate Email
Validate URL
Input length

1. Required Field Validation: This ensures that a form field is not empty or null.
Syntax:
if(empty($_POST[‘field_name’])) {
$errors[] = ‘Field is required’;
}
Example :-
<form method=”post” action=””>
<label for=”name”>Name:</label>
<input type=”text” name=”name” id=”name”>
<button type=”submit”>Submit</button>
</form>

<?php
if($_SERVER[‘REQUEST_METHOD’] == ‘POST’) {
if(empty($_POST[‘name’])) {
$errors[] = ‘Name is required’;
}
}
?>

2. Email Validation: This ensures that an email field is a valid email format.
Syntax:
if(!filter_var($_POST[’email’], FILTER_VALIDATE_EMAIL)) {
$errors[] = ‘Invalid email format’;
}
Example:
<form method=”post” action=””>
<label for=”email”>Email:</label>
<input type=”email” name=”email” id=”email”>
<button type=”submit”>Submit</button>
</form>

<?php
if($_SERVER[‘REQUEST_METHOD’] == ‘POST’) {
if(!filter_var($_POST[’email’], FILTER_VALIDATE_EMAIL)) {
$errors[] = ‘Invalid email format’;
}
}
?>
3. Numeric Validation: This ensures that a field contains only numeric values.
Syntax
if(!is_numeric($_POST[‘age’])) {
$errors[] = ‘Age must be numeric’;
}
Example :-
<form method=”post” action=””>
<label for=”age”>Age:</label>
<input type=”text” name=”age” id=”age”>
<button type=”submit”>Submit</button>
</form>

<?php
if($_SERVER[‘REQUEST_METHOD’] == ‘POST’) {
if(!is_numeric($_POST[‘age’])) {
$errors[] = ‘Age must be numeric’;
}
}
?>

4.Password Validation: This ensures that a password field meets certain criteria, such as minimum length and use of special characters.
Syntax:
if(strlen($_POST[‘password’]) < 8) {
$errors[] = ‘Password must be at least 8 characters long’;
}
<form method=”post” action=””>
<label for=”password”>Password:</label>
<input type=”password” name=”password” id=”password”>
<button type=”submit”>Submit</button>
</form>

<?php
if($_SERVER[‘REQUEST_METHOD’] == ‘POST’) {
if(strlen($_POST[‘password’]) < 8) {
$errors[] = ‘Password must be at least 8 characters long’;
}
}
?>

5.File Upload Validation: This ensures that a file upload meets certain criteria, such as file type and file size.
Syntax:
if($_FILES[‘file’][‘type’] != ‘image/jpeg’ || $_FILES[‘file’][‘size’] > 500000) {
$errors[] = ‘Invalid file format or file size too large’;
}
Example:
<form method=”post” action=”” enctype=”multipart/form-data”>
<label for=”file”>File:</label>
<input type=”file” name=”file” id=”file”>
<button type=”submit”>Submit</button>
</form>

<?php
if($_SERVER[‘REQUEST_METHOD’] == ‘POST’) {
if($_FILES[‘file’][‘type’] != ‘image/jpeg’ || $_FILES[‘file’][‘size’]