php where Clause

Pattern:
SELECT column_name(s)
FROM table_name
WHERE column_name operator value

PHP code Example:
$myquery= mysql_query("SELECT * FROM Persons
WHERE FirstName='John' ");

$result=mysql_query($myquery);
if(!$result)
{
$error = "Query fail!";
return($error);
}

Where Clause with variable
$myquery ="select * from userdb where user = '".$myuser."' and usepw= '".$mepw."' ";

this clause specific 2 variables those are:

$myuser => ' " .$myuser. " '

$mepw => ' " .$mepw. " '

Please Note: ' " .$myuser. " ' => must no space => should be '".$myuser."'