Array search

hi,
how to search the string value in array.
ex: if my array contains
$ar1=array(“activate”=>“1”,“activated”=>“3”,“provide”=>“1”,“activating”=>“2”,“providing”=>“3”,“provided”=>“2”,“hello”=>“6”);
then how to search the string word activate exists in array or not
from the above array it has search for activate and display the result as 3 times bcz activate is there exists three rimes. how to find