PHP: Type conversion

Derek Bridge

Department of Computer Science,
University College Cork

PHP: Type conversion

Aims:

• to learn how PHP converts a value from one type to another
• to learn the difference between explicit type conversion and implicit type conversion

Explicit type conversion

• We saw how to convert the user's data (strings) to integers and floats, e.g.:

  $num_of_students = (int) $_GET['num_of_students'];
  $num_of_staff = (int) $_GET['num_of_staff'];
  
  $num_of_students_per_staff = $num_of_students / $num_of_staff;
     
  echo '<p>';
  echo $num_of_students_per_staff;
  echo '</p>';

Implicit type conversion

• Suppose you do not explicitly request type conversion, e.g.:

  $num_of_students = $_GET['num_of_students'];
  $num_of_staff = $_GET['num_of_staff'];
  
  $num_of_students_per_staff = $num_of_students / $num_of_staff;
     
  echo '<p>';
  echo $num_of_students_per_staff;
  echo '</p>';

  Asking PHP to divide one string by another doesn't make sense!

Implicit type conversion

• In some programming languages,
  • you get a fatal error (at compile-time or run-time, depending on the programming language) for trying to apply an operator to operands of the wrong type
• But in PHP
  • this gives rise to no error message nor warning message
  • PHP carries out some type conversions for you, when it needs to
• Advice: think hard and be self-disciplined about types
  • hence, use explicit type conversion when you need to convert user strings to numeric data

Converting numeric data to strings

• Above we saw conversions from strings to numbers: for input data
• But we often want to convert numbers to strings: for output
• Concatenation and interpolation operate on strings
• This is a classic case where we will use implicit type conversion

Example with concatenation

$num_of_students = (int) $_GET['num_of_students'];
$num_of_staff = (int) $_GET['num_of_staff'];

$num_of_students_per_staff = $num_of_students / $num_of_staff;
   
echo '<p>The staff-student ratio in your college is: ' . $num_of_students_per_staff . '</p>';

Example with interpolation

$num_of_students = (int) $_GET['num_of_students'];
$num_of_staff = (int) $_GET['num_of_staff'];

$num_of_students_per_staff = $num_of_students / $num_of_staff;
   
echo "<p>The staff-student ratio in your college is: {$num_of_students_per_staff}</p>";

Class exercise

• Suppose age_form.html contains this form:

  <form action="next_age.php" method="get">
      <input type="text" name="firstname" />
      <input type="text" name="age" value="0" />
      <input type="submit" />
  </form>

• Write next_age.php which tells the user his/her age next birthday
• E.g. in my case, the output will be the following paragraph:

  Derek, on your next birthday you will be 29

PHP's type conversions: summary

(Lots of little details suppressed here, e.g. what if you try to convert something that's too big)

Class exercise

• HotFlush is a wholesaler which sells curry powder in small 25kg bags and large 50kg bags
• A user can order curry powder using this form:

  <form action="curry_powder_order.php" method="get">
      <input type="text" name="num_of_small_bags" value="0" />
      <input type="text" name="num_of_large_bags" value="0" />
      <input type="submit" />
  </form>

• Write curry_powder_order.php which outputs an unordered list, like the following for a user who ordered 3 of the smaller bags and 2 of the larger bags:

  • 3 * small bags: 75kg
  • 2 * large bags: 100kg
  • Total weight: 175kg