PHP CookBook

Recipe 6.5 Using Named Parameters

6.5.1 Problem

You want to specify your arguments to a function by name, instead of simply their position in the function invocation.

6.5.2 Solution

Have the function use one parameter but make it an associative array:

function image($img) {
    $tag  = '<img src="' . $img['src'] . '" ';
    $tag .= 'alt="' . ($img['alt'] ? $img['alt'] : '') .'">';
    return $tag;
}

$image = image(array('src' => 'cow.png', 'alt' => 'cows say moo'));
$image = image(array('src' => 'pig.jpeg'));

6.5.3 Discussion

While using named parameters makes the code inside your functions more complex, it ensures the calling code is easier to read. Since a function lives in one place but is called in many, this makes for more understandable code.

When you use this technique, PHP doesn't complain if you accidentally misspell a parameter's name, so you need to be careful because the parser won't catch these types of mistakes. Also, you can't take advantage of PHP's ability to assign a default value for a parameter. Luckily, you can work around this deficit with some simple code at the top of the function:

function image($img) {
    if (! isset($img['src']))    { $img['src']    = 'cow.png';      }
    if (! isset($img['alt']))    { $img['alt']    = 'milk factory'; }
    if (! isset($img['height'])) { $img['height'] = 100;            }
    if (! isset($img['width']))  { $img['width']  = 50;             }
    ...
}

Using the isset( ) function, check to see if a value for each parameter is set; if not, assign a default value.

Alternatively, you can write a short function to handle this:

function pc_assign_defaults($array, $defaults) {
    $a = array( );
    foreach ($defaults as $d => $v) {
        $a[$d] = isset($array[$d]) ? $array[$d] : $v;
    }

    return $a;
}

This function loops through a series of keys from an array of defaults and checks if a given array, $array, has a value set. If it doesn't, the function assigns a default value from $defaults. To use it in the previous snippet, replace the top lines with:

function image($img) {
    $defaults = array('src'    => 'cow.png',
                      'alt'    => 'milk factory',
                      'height' => 100,
                      'width'  => 50
                     );
    $img = pc_assign_defaults($img, $defaults);
    ...
}

This is nicer because it introduces more flexibility into the code. If you want to modify how defaults are assigned, you only need to change it inside pc_assign_defaults( ) and not in hundreds of lines of code inside various functions. Also, it's clearer to have an array of name/value pairs and one line that assigns the defaults instead of intermixing the two concepts in a series of almost identical repeated lines.

6.5.4 See Also

Recipe 6.6 on creating functions that accept a variable number of arguments.