That is one of the bitwise operators supported by php, documented at http://php.net/manual/en/language.operators.bitwise.php
The ampersand is a bitwise AND. basically it looks at the binary representations of the operands, and returns true if one is contained in the other. In the example you've given, I'm assuming $strength is either a boolean value that is currently set to true, or it's an odd number. here's a drawn out example assuming the latter.
if $strength contained the number 33...
$strength's binary representation would be 100001 a bitwise and of that and 1 (000001) shows that they both have 1 in the 2^0 spot.
There are more examples in the manual, and I found an old example I gave someone on another forum forever ago on bitwise 'or' operations.
http://www.autoitscript.com/forum/topic ... l__bitwise