string integer error

I am receiving this error

Warning: array_key_exists() [function.array-key-exists]: The first argument should be either a string or an integer in /home/dhs303/public_html/wp-includes/category-template.php on line 176

Here is the code.

$categories = get_object_term_cache($post->ID, ‘category’);
if ( false === $categories )
$categories = wp_get_object_terms($post->ID, ‘category’);
if(array_key_exists($category, $categories))
return true;
else
return false;

Can anyone tell me what the problem is