array array_keys(array input, mixed
[search_value]);array_keys() returns the keys, numeric and string, from the input array.
If the optional search_value is specified, then only the keys for that value are returned. Otherwise, all the keys from the input are returned.
Example 1. array_keys() example $array = array(0 => 100, "color" => "red");
array_keys($array); // returns array(0, "color")
$array = array(1, 100, 2, 100);
array_keys($array, 100); // returns array(0, 2)
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See also array_values().
Note: This function was added in PHP 4.0.