<?php
$text = 'bla bla bla bla (=abc) bla bla bla bla (=var.var)';
$text = preg_replace_callback('/\(=([\w.]+?)\)/', 'processMatches', $text);
function processMatches($matches) {
$array = array();
$array['abc']='word1';
$array['var.var']='word2';
return isset($array[$matches[1]]) ? $array[$matches[1]] : '';
}
var_dump($text);