session - php prepared statement inserting with trim or date() etc. php strict standards error -

hopefully easy question, haven't been able find answer. i'm learning use prepared statements rather mysqli_escape. have code:

$stmt = $dbc->prepare("select the_table email=? "); $stmt->bind_param("s", strtolower(trim($_request['email']))); 

i error message "php strict standards: variables should passed reference in".

am correct in thinking you're not supposed use strtolower / trim etc in bind param line? important? less secure first have separate:

   $email = strtolower(trim($_request['email']))); 

i kind of thought should try , keep $_post, $_request bits in bind_param line.

i same issue in page with:

$stmt->bind_param("s", date("y-m-d") ); 

lastly , separately, safe use insert $_session variable directly? these have been set previously, can hacked? if i've set $_session['admin']="off" earlier, , later use in query admin=? bind_param says ("s", $_session['admin']); safe?

many thanks.

when trim($x), comes out not variable, reference value. bind_param method expects variable work. so, means need formatting , other calls before pass function. yes, thinking correct.

for example correct way it:

$date = date('y-m-d'); $stmt->bind_param("s", $date); 

i recommend read on references better understanding: http://php.net/manual/en/language.references.php