Q:

What does =& mean in PHP?

Consider:
$smarty =& SESmarty::getInstance();

What is the & for?

A:

It passes by reference. Meaning that it won't create a copy of the value passed.
See:
http://php.net/manual/en/language.references.php (See Adam's Answer)
Usually, if you pass something like this:
$a = 5;
$b = $a;
$b = 3;

echo $a; // 5
echo $b; // 3

The original variable ($a) won't be modified if you change the second variable ($b) . If you pass by reference:
$a = 5;
$b =& $a;
$b = 3;

echo $a; // 3
echo $b; // 3

The original is changed as well. 
Which is useless when passing around objects, because they will be passed by reference by default.

A:

In PHP 4, it kind of (awkwardly) associated two variables.
$j = 'original';
$i =& $j;
$i = 'modified';
echo $j; // The output is 'modified'

Likewise...
$j = 'original';
$i =& $j;
$j = 'modified';
echo $i; // The output is 'modified'

Some of this was made a little less unpleasant when it comes to objects in PHP 5, but I think the heart of it is the same, so these examples should still be valid.

