Form Validation / Required

An HTML form contains various input fields such as text box, checkbox, radio buttons, submit button, and checklist, etc. These input fields need to be validated, which ensures that the user has entered information in all the required fields and also validates that the information provided by the user is valid and correct.

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:

1. Input length
2. Empty String
3. Validate String
4. Validate Numbers
5. Validate Email
6. Validate URL

Input Length Validation

The input length validation restricts the user to provide the value between the specified range, for Example - Mobile Number. A valid mobile number must have 10 digits.

The given code will help you to apply the length validation on user input:

$mobileno = strlen ($_POST ["Mobile"]);  
$length = strlen ($mobileno);  
  
if ( $length < 10 && $length > 10) {  
    $ErrMsg = "Mobile must have 10 digits.";  
            echo $ErrMsg;  
} else {  
    echo "Your Mobile number is: " .$mobileno;  
}  

Empty String

The code below checks that the field is not empty. If the user leaves the required field empty, it will show an error message. Put these lines of code to validate the required field.

if (emptyempty ($_POST["name"])) {  
    $errMsg = "Error! You didn't enter the Name.";  
             echo $errMsg;  
} else {  
    $name = $_POST["name"];  
}  

Validate String

The code below checks that the field will contain only alphabets and whitespace, for example - name. If the name field does not receive valid input from the user, then it will show an error message:

$name = $_POST ["Name"];  
if (!preg_match ("/^[a-zA-z]*$/", $name) ) {  
    $ErrMsg = "Only alphabets and whitespace are allowed.";  
             echo $ErrMsg;  
} else {  
    echo $name;  
}  

Validate Number

The below code validates that the field will only contain a numeric value. For example - Mobile no. If the Mobile no field does not receive numeric data from the user, the code will display an error message:

$mobileno = $_POST ["Mobile_no"];  
if (!preg_match ("/^[0-9]*$/", $mobileno) ){  
    $ErrMsg = "Only numeric value is allowed.";  
    echo $ErrMsg;  
} else {  
    echo $mobileno;  
}  

Validate Email

A valid email must contain @ and . symbols. PHP provides various methods to validate the email address. Here, we will use regular expressions to validate the email address.

The below code validates the email address provided by the user through HTML form. If the field does not contain a valid email address, then the code will display an error message:

$email = $_POST ["Email"];  
$pattern = "^[_a-z0-9-]+(\.[_a-z0-9-]+)*@[a-z0-9-]+(\.[a-z0-9-]+)*(\.[a-z]{2,3})$^";  
if (!preg_match ($pattern, $email) ){  
    $ErrMsg = "Email is not valid.";  
            echo $ErrMsg;  
} else {  
    echo "Your valid email address is: " .$email;  
}  

Validate URL

The below code validates the URL of website provided by the user via HTML form. If the field does not contain a valid URL, the code will display an error message, i.e., "URL is not valid".

$websiteURL = $_POST["website"];  
if(!preg_match("/\b(?:(?:https?|ftp):\/\/|www\.)[-a-z0-9+&@#\/%?=~_|!:,.;]*[-a-z0-9+&@#\/%=~_|]/i",$website)) 
{  
  $websiteErr = "URL is not valid";  
  echo $websiteErr;  
} 
else 
{  
    echo "Website URL is: " .$websiteURL;  
}