query category

/* Pull category for each unique post using the ID 
*/
$terms = get_the_terms( $post->ID, 'project_categories' );    
    if ( $terms && ! is_wp_error( $terms ) ) : 
$links = array();
foreach ( $terms as $term ) {
$links[] = $term->name;
}
$tax_links = join( " ", str_replace(' ', '-', $links));          
$tax = strtolower($tax_links);
else :    
$tax = '';                    
endif;


//or for when you need to do a str_replace on more than one item

    $search = array(' ', '&', 'amp;');
    $replace = array('-', 'and','');

    $tax_links = join( " ", str_replace($search, $replace, $links));
    $tax = strtolower($tax_links);