PHP: Variables

Derek Bridge

Department of Computer Science,
University College Cork

PHP: Variables

Aims:

to learn about variables and assignment in PHP
to learn how to initialise variables with external data

Variables in PHP

In programming, variables are named storage locations
You can store strings, integers, floating-point numbers,...
You use a variable to hold intermediate results, data received from the user,...
In PHP, the name (or identifier)
- is preceded by a dollar sign, $
- then a letter or underscore, _
- then zero, one or more letters, digits and/or underscores
- is case-sensitive, e.g. $qty, $QTY and $Qty are different
But above all, choose a name that is meaningful

`answer1.php`: a script that uses a variable

<body>

    <?php 
        $answer = 'Forty-two.'; 
        echo '<p>What is the answer to Life, the Universe, and Everything?</p>';
        echo '<p>';
        echo $answer; 
        echo '</p>';
    ?>

</body>

Assignment

To assign a value to a variable (i.e. to store something), use =

$answer = 'Forty-two.';
The left-hand side is always the variable (where to store)
The right-hand side is the value (what to store), which can be
- a simple value, e.g. $answer = 'Forty-two.';
- another variable, e.g. $my_age = $answer;
- a more complex expression, e.g. $greeting = 'Hello ' . 'world';

Initialisation

The first statement to assign a value to a variable initialises that variable ('initialisation')
But you can store different values in the variable over the course of the program (it can vary, hence 'variable')

`answer2.php`: another script that uses a variable

<body>

    <?php 
        $answer = 'You\'re not going to like it.';
        echo '<p>What is the answer to Life, the Universe, and Everything?</p>';
        echo '<p>';
        echo $answer;
        echo '</p>';
        $answer = 'Forty-two.';
        echo '<p>What is the answer to Life, the Universe, and Everything?</p>';
        echo '<p>';
        echo $answer;
        echo '</p>';
    ?>
 
</body>

This is pointless!

So far, each of our programs is pointless: every time you run it, you get the same output
This is because all the data is hard-coded into the program
Class exercise: In fact, one program wasn't like this. Which one?
For more interesting programs, we need the ability to obtain external data, e.g. from a database, from the user,...

Initialising variables with form data

Suppose form.html contains this form:

<form action="process.php" method="get">
    <input type="text" name="firstname" />
    <input type="text" name="surname" />
    <input type="submit" />
</form>

Suppose the user enters 'Hugh' and 'Jeegoh' and presses Submit
The browser sends this request
GET process.php?firstname=Hugh&surname=Jeegoh

`process.php`: a script that greets you by name

<body>

    <?php
        $firstname = $_GET['firstname'];
        $surname = $_GET['surname'];
        
        echo '<p>';
        echo 'Hello ';
        echo $firstname;
        echo ' ';
        echo $surname;
        echo '</p>';
   ?>
 
</body>

Notes on `process.php`

$_GET allows you to access the data that was entered into the form and sent in the request
Inside the square brackets, you use the value of the name attribute from the form
We have used this to initialise two PHP variables

Class exercise

Suppose home.html contains this form:

<form action="display_home.php" method="get">
    <input type="text" name="town" />
    <input type="text" name="county" />
    <input type="submit" />
</form>

Write display_home.php, which displays a paragraph such as (in my case):
You are from Lowestoft in Suffolk.

PHP: Variables

Derek Bridge

Department of Computer Science, University College Cork

PHP: Variables

Variables in PHP

answer1.php: a script that uses a variable

Assignment

Initialisation

answer2.php: another script that uses a variable

This is pointless!

Initialising variables with form data

process.php: a script that greets you by name

Notes on process.php

Class exercise

Department of Computer Science,
University College Cork

`answer1.php`: a script that uses a variable

`answer2.php`: another script that uses a variable

`process.php`: a script that greets you by name

Notes on `process.php`