update problem. please help

[escalador]

I need a little help please

Like this it's working

$sql = "UPDATE users SET city='$_POST[city]' WHERE username='escalador'";
$result = mysql_query($sql);

Like this it's working

$sql = 'UPDATE users SET city="L.A." WHERE username="'.$_SESSION["username"].'";';

And now don't work... 

$sql = "UPDATE users SET city='$_POST[city]' WHERE username="'.$_SESSION["username"].'";";
$result = mysql_query($sql);

and gives this error

Parse error: syntax error, unexpected T_CONSTANT_ENCAPSED_STRING in C:\xampp\htdocs\change.php on line 15

What is the problem ?

Thank's

[Symmetry]

$sql = "UPDATE users SET city='".$_POST[city]."' WHERE username='".$_SESSION["username"]."'";

[escalador]

thank's it's working