5.2. Establishing a Default Value
Problem
You want to assign a default value to a variable that doesn’t already have a value. It often happens that you want a hardcoded default value for a variable that can be overridden from form input or through an environment variable.
Solution
Use isset( )
to assign a default to a variable
that may already have a value:
if (! isset($cars)) { $cars = $default_cars; }
Use the
ternary (a ? b : c
)
operator to give a new variable a (possibly default) value:
$cars = isset($_REQUEST['cars']) ? $_REQUEST['cars'] : $default_cars;
Discussion
Using isset( )
is essential when assigning default
values. Without it, the nondefault value can’t be
0
or anything else that evaluates to
false
. Consider this assignment:
$cars = $_REQUEST['cars'] ? $_REQUEST['cars'] : $default_cars;
If $_REQUEST['cars']
is 0
,
$cars
is set to $default_cars
even though 0
may be a valid value for
$cars
.
You can use an array of defaults to set multiple default values easily. The keys in the defaults array are variable names, and the values in the array are the defaults for each variable:
$defaults = array('emperors' => array('Rudolf II','Caligula'), 'vegetable' => 'celery', 'acres' => 15); foreach ($defaults as $k => $v) { if (! isset($GLOBALS[$k])) { $GLOBALS[$k] = $v; } }
Because the variables are set in the global namespace, the previous code doesn’t work for setting function-private defaults. To do that, use variable variables:
foreach ($defaults as $k => $v) { if (! isset($$k)) { ...
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