in the current (CVS) version of PHP4, this function
is definend in php4/ext/standard/basic_functions.c
and implemented in php4/ext/standard/array.c

it is documented in array.xml

a japanese translation is available

this function lacks a proto definition in latest PHP3 source

there is a proto in latest PHP4 source:

array array_keys(array input [, mixed search_value]) Return just the keys from the input array, optionally only for the specified search_value