PHP: Parameter Passing

Derek Bridge

Department of Computer Science,
University College Cork

PHP: Parameter Passing

Aims:

• to learn what is meant by pass-by-value and pass-by-reference
• to learn when to use each of these

Common error

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

  function increase_prices( $product_prices, $increase )
  { 
      foreach($product_prices as $product => $price)
      {
          $product_prices[$product] = $price * (1 + $increase);
      }
  }
  
  $fruit_prices = array('Apples' => 1.59, 'Pears' => 2.34,
                    'Kumquats' => 4.05, 'Jujubes' => 2.34);
  increase_prices($fruit_prices, 0.1);
  echo "<ul>";
  foreach($fruit_prices as $fruit => $price)
  {
      echo "<li>{$fruit}: {$price}</li>";
  }
  echo "</ul>";

Simpler example of the same error

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

  function celebrate_birthday( $age )
  {
      $age++;
  }
  
  $my_age = 20;
  celebrate_birthday( $my_age );
  echo $my_age;

Pass-by-value

• As we know, when a function is invoked, temporary variables are created for each formal parameter
• By default, copies of the values of the actual parameters are placed into these temporary variables in order to initialise them
• This is known as pass-by-value (or sometimes 'call-by-value')

Pass-by-value

• In the lecture, we will work through this example to illustrate pass-by-value

  function square( $x )
  {
      return $x * $x;
  }
  
  $v = 3;
  echo '<p>' . square($v) . '</p>';

Pass-by-value

• It follows that in pass-by-value any changes a function makes to its formal parameters have no effect on the corresponding actual parameters (because the function is working on copies!)
• Now we can understand why the two erroneous programs from earlier do not behave as the programmer intended
• In the lecture, make sure you understand this fully
• Class exercise: Using only what we've covered so far, fix the two erroneous programs

Pass-by-reference

• In PHP, when actual parameters are variables it is possible for the programmer to request that their addresses, rather than their values, are used to initialise the formal parameters
• This is known as pass-by-reference (or sometimes 'call-by-reference')
• You request this in the function's parameter list by prefixing an ampersand (&) on the formal parameter

Pass-by-reference

• In the lecture, we will work through this example to illustrate pass-by-reference

  function celebrate_birthday( &$age )
  {
      $age++;
  }
  
  $my_age = 20;
  celebrate_birthday( $my_age );
  echo $my_age;

Pass-by-reference

• It follows that in pass-by-reference any changes a function makes to its formal parameters do effect the corresponding actual parameters (because the addresses 'point' to the originals)
• In the lecture, make sure you understand this fully
• Class exercise: Use pass-by-reference parameters to fix the other erroneous program

When to use pass-by-reference

• Advice: use sparingly
• When your function needs direct access to a variable in order to modify its value
  • this is a classic case to use pass-by-reference
  • but we saw an alternative: return the modified value
• When your actual parameter is a very long string or very large array
  • using pass-by-reference saves time and memory
  • if we use pass-by-value, the whole string/array would need to be copied into the formal parameter

Observation

• In some languages, such as PHP, the programmer chooses between pass-by-value (default) and pass-by-reference (e.g. by using &)
• In a language such as Java, the decision is taken out of the programmers hands:
  • simple values (integers, floats, Booleans,...) are all pass-by-value
  • strings, arrays (and other objects) are all, in some sense, pass-by-reference (without using &)

Pointers for advanced students

• By default in PHP, return values are copied out of a function
• By prefixing the function name with &, the address of the value is returned
• This facility might be useful if the return value is a long string or large array
• But in fact PHP has certain optimisations ('copy-on-write') that mean that this facility is rarely necessary