Dynamic SQL queries

i have a drop down list menu in one of form and when the user selects a category i need to write a sql statement to get the details from the catergory selected.
Eg:
When the user selects option 1,the table 1 should be displayed
if the user selects option 2 the table 2 must be displayed.

i need to know is there a way that i can pass the option value to the sql query rathe than using if else statemets

like,
select * from <option selected>

 

In PHP:

$query = "SELECT * FROM " . mysql_real_escape_string($optionselected);

 

That doesn't work

$result = mysql_query("SELECT * FROM members WHERE last=$last AND first=$first");

i need to make the members dynamic..

if the user selects 1 load the table 1 and display the content across the complete site
if the user selects 2,load the table 2 and display the content across the complete site

 

$query = "SELECT * FROM " . mysql_real_escape_string($optionselected) . "WHERE last=$last AND first=$first";
$result = mysql_query($query);

PHP:

Why doesn't that work?