PHP: One-Armed Conditional Statements II

Derek Bridge

Department of Computer Science,
University College Cork

PHP: One-Armed Conditional Statements II

Aims:

• to learn common errors involving one-armed conditional statements
• to learn how to use radio buttons

Class exercise

• Each of the next five slides shows an error, many of them common errors
• In each case,
  • what is the error?
  • will PHP give an error/warning message?
  • if so, will it be a compile-time or run-time error/warning message?
  • how do we fix it?

Common errors I

if $age < 18 
{
    echo 'You are too young to study CS1109';
    echo 'Come back when you\'re older.';
}

Common errors II

if ( $age < 18 );
{
    echo 'You are too young to study CS1109';
    echo 'Come back when you\'re older.';
}

Common errors III

if ( $age < 18 ) 
    echo 'You are too young to study CS1109. ';
    echo 'Come back when you\'re older.';

Common errors IV (probable error)

if ( $age < 18 )
{ 
    echo 'You are too young to study CS1109. ';
    echo 'Come back when you\'re older.';
}
if ( $age > 18 )
{
    echo 'Welcome to CS1109.';
}

Common errors V

if ( $age = 18 ) 
{
    echo 'You are the perfect age to study CS1109.';
}

Explaining common error V, part I: assignment

• Revision of operators: 2 * 3 evaluates to 6
• Assignment (=) is also an operator
  • Not only does it store a value in a variable
  • It also returns that value
• E.g. $age = 18
  • stores 18 in variable $age
  • returns 18

Explaining common error V, part I: assignment

• Class exercise: What is the output of each of the following? Also, what do the variables contain afterwards?
  1. echo $age = 18;
  2. echo ($my_age = 19) + 1;
• There are one or two places where we exploit this idea but...
• Advice: generally do not exploit this idea

Explaining common error V, part II: Boolean values

• The Boolean data type has just two values: false and true
• But recall that PHP often silently carries out conversions from values of one type to values of another type

Explaining common error V, part II: Boolean values

• Class exercise: What is the output of the following:

  $x = 1;
  $y = '';
  if ( $x )
  {
      echo 'Whoo hoo!';
  }
  if ( $y )
  {
      echo 'Whey hey!';
  }
  if ( $z )
  {
      echo 'Yee haa!';
  }

Can we now explain common error V?

if ( $age = 18 ) 
{
    echo 'You are the perfect age to study CS1109.';
}

HTML for radio buttons

<form action="colour.php" method="get">
   <fieldset>
      <legend>Guess my favourite colour</legend>
      <div>
         <input type="radio" name="colour" value="red" id="red" checked="checked" />
         <label for="red">Red</label>
         <input type="radio" name="colour" value="green" id="green" />
         <label for="green">Green</label>
         <input type="radio" name="colour" value="blue" id="blue" />
         <label for="blue">Blue</label>
      </div>
   </fieldset>
    
   ...submit button, etc...
    
</form>

HTML for radio buttons

• For radio buttons: <input type="radio" />
• Usually you have a group of radio buttons
  • the user can check only one of the group at a time
  • give each member of the group the same name
  • give each member of the group a different value
  • it is common to use checked="checked" on one button so that it is already pre-selected
• If the user selects a button and submits, that button's name and value are sent to the server

PHP for radio buttons

$colour = $_GET['colour'];

$winnings = 0;
if ( $colour == 'green' )
{
    $winnings = 5;
}
echo "<p>You win {$winnings} eurines!</p>";