Client validation: The input data validations are performed on the client machine's web browsers using client side script like Java script or additional "required" attribute in HTML input tags.
Before sending the data to server side program (PHP) the programmer can write few validation from browser in the client machine. For this validation, we have to add additional "required" attribute in HTML in input tag.
Example:
<input> required attribute in HTML
<form action = "welcome.php">
Username: <input type = "text" name = "name" required>
<input type = "submit">
</form>
Output

Test.html
< html >
< body >
< form action = "welcome.php" method = "post" >
Name: < input type = "text" name = "name" > < br >
Email < input type = "text" name = "email" > < br >
< input type = "submit" >
< /form >
< /body >
< /html >
Welcome,php
< html >
< bodp >
Welcome <?php echo $_POST["name"]:
?> <br>
Your email address is: <?php echo $_POST ["email"];
?>
</body>
</html>
Output:

Explanation:
(i) HTML file contains two text boxes such as Name and Email, one Submit button and one form tag. The remote server PHP file (welcome.php) is mentioned in form tag under the action attribute.
(ii) In "Welcome.php" file, PHP variables such as $_POST and $_GET collect the data and prepares the response accordingly. The user will receive the output response in the client machine's browser screen.

1. File upload is the best feature to select one file from the local machine to server machine.
2. Form tag is used to mention a method as POST or GET and encrypt attribute mentioned as "multipart/form-data".
3. In the < Input > tag mention type="file" attribute shows the input field as a file-select control, with a "Browse" button next to the input control.
4. The form above sends data to a file.
5. In Server machine "php.ini" file, search for the file_uploads directive, and set it to On: "file_uploads = On"
6. After submitting the upload button the request reaches to the file.
7. In the file ＄FILES variable collects all uploaded file information such as name of the file, size of the file and extension of the file etc.
8. All the details are checked thoroughly and the errors are saved in an array variable.
9. The file finally moves under the image directory if the array error variable is empty.