Form Validation

User input should be validated on the browser whenever possible (by client scripts). Browser validation is faster and reduces the server load.
You should consider server validation if the user input will be inserted into a database. A good way to validate a form on the server is to post the form to itself, instead of jumping to a different page. The user will then get the error messages on the same page as the form. This makes it easier to discover the error.
The built-in $_GET function is used to collect values in a form with method="get".
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The $_GET Function
The built-in $_GET function is used to collect values from a form sent with method="get".
Information sent from a form with the GET method is visible to everyone (it will be displayed in the browser's address bar) and has limits on the amount of information to send (max. 100 characters).
Example
< form action="welcome.php" method="get" >
Name: < input type="text" name="fname" / >
Age: < input type="text" name="age" / >
< input type="submit" / >
< /form >
When the user clicks the "Submit" button, the URL sent to the server could look something like this:
http://www.w3schools.com/welcome.php?fname=Peter&age=37
The "welcome.php" file can now use the $_GET function to collect form data (the names of the form fields will automatically be the keys in the $_GET array):
Welcome < ?php echo $_GET["fname"]; ?>.

You are < ?php echo $_GET["age"]; ? > years old!