php - preg_replace when not inside double quotes

Basically I want to replace certain words (e.g. the word
"tree" with the word "pizza") in sentences. Restriction: When the word that should be
replaced is between double quotes, the replace should not be
performed.

Example:

The
            tree is green. -> REPLACE tree WITH pizza

"The" tree is "green".
            -> REPLACE tree WITH pizza
"The tree" is green. -> DONT
            REPLACE
"The tree is" green. -> DONT REPLACE
The ""tree is green.
            -> REPLACE tree WITH
            pizza

Is it possible
to do this with regular expressions? I would count the number of double quotes before
the word and check if it is odd or even. But is this possible using preg_replace in
php?

Thanks!

//EDIT:

At
the moment my code looks like the
following:

preg_replace("/tree/",
            "pizza",
            $sentence)

But the
problem here is to implement the logic with the double quotes. I tried things
like:

preg_replace("/[^"]tree/",
            "pizza",
            $sentence)

But
this does not work, because it checks only if a double quote is in front of the word.
But there are examples above where this check fails.
Import is that I want to
solve that problem with regex only.