$sql = (
  'cmd1'=>"select * from table1 where user_id = $user_id",
  'cmd2'=>"insert into table2(fld1,fld2) values ('$field_code','new')"
);
...
if ($action=='cmd1') {
  $user_id = 1;
  mysql_query($sql['cmd1']);
} elseif ($action=='cmd2') {
  $field_code = 'test';
  mysql_query($sql['cmd2']);
}
...

function db_query($cmd, $args)
{
	global $dbh, $sql;
	$s = $sql[$cmd];
	if (preg_match_all('#\$([a-zA-Z0-9_]+)#', $s, $matches, PREG_SET_ORDER));
	{
		foreach($matches as $m)
		{
			$s = str_replace($m[0], $args[$m[1]], $s);
		}
	}
	if ($result = $dbh->query($s))
	{
		...
	}
}

function return_sql_cmd1($arg1, $arg2) {
  $res = "select * from table where fld1 = $arg1 and fld2 = '$arg2'";
return $res}
...
mysql_query(return_sql_cmd1(1,'test'));