Query Strings

http://php.net/manual/en/

Using Query Strings:
Query strings are variable name/value pairs. Example:

www.example.com/page.php?fname=John&lname=Doe

The file "page.php" is being passed variables, along with their respective values, using a query string. Here, the variables "fname" and "lname" are being passed, with values "John" and "Doe," respectively.

1. question mark (?) indicates beginning of query string
2. variable_name=value assignments follow question mark
3. ampersand (&) symbol separates variable_name=value assignments
4. requested page able to read variable/value pairs, and use them accordingly
5. query strings are accessed using superglobal array $_GET

Example:

www.example.com/page.php?fname=John&lname=Doe

//access values of variables fname and lname in $_GET associative array (key/value pairs)
echo $_GET['fname']; //displays "John" (w/o quotation marks)
echo $_GET['lname']; //displays "Doe" (w/o quotation marks)

//or assign value local variable
$cusFname = $_GET['fname']; 
$cusLname = $_GET['lname']; 

perform some action on variables...

Example: Query String Process

<form method="GET" action="page.php">
First Name: <input type="text" name="fname" />
Last Name: <input type="text" name="lname" />
<input type="submit" value="submit" />
</form>

Example:

First Name:
Last Name:

Two ways to pass variables to SQL queries:

1) Select * FROM customer WHERE cus_last='" . $_GET['lname'] . "' 

2) 
$cusLname = $_GET['lname'];
Select * FROM customer WHERE cus_last='$cusLname' 

Note:
Difference in (') and ('") in examples above.

Note:
Can use POST/GET/REQUEST

References:
Using query strings in PHP
How to use the query string in PHP