Programming PHP

The default argument, if provided, is a seed value. For instance, if we change the call
to array_reduce( ) in the previous example to:
$total = array_reduce($numbers, 'add_up', 11);
The resulting function calls are:
add_up(11,2)
add_up(13,3)
add_up(16,5)
add_up(21,7)
If the array is empty, array_reduce( ) returns the default value. If no default value is
given and the array is empty, array_reduce( ) returns NULL.
Searching for Values
The in_array( ) function returns true or false, depending on whether the first argument
is an element in the array given as the second argument:
if (in_array(to_find, array [, strict])) { ... }
If the optional third argument is true, the types of to_find and the value in the array
must match. The default is to not check the types.
Here’s a simple example:
$addresses = array('spam@cyberpromo.net', 'abuse@example.com',
'root@example.com');
$got_spam = in_array('spam@cyberpromo.net', $addresses); // $got_spam is true
$got_milk = in_array('milk@tucows.com', $addresses); // $got_milk is false
PHP automatically indexes the values in arrays, so in_array( ) is much faster than a
loop that checks every value to find the one you want.
Example 5-2 checks whether the user has entered information in all the required
fields in a form.
Example 5-2. Searching an array
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