Здравствуйте. В переменной $text хранится кучя текста. Как подсветить конкретные слова ? например '(ah-sah-ee)';

$lookingFor = "(ah-sah-ee)";
$text = "Acai (Euterpe olereacea), pronounced (ah-sah-ee) is a fruit that grows on the Acai Palm Trees in the Amazonian Rainforest of Brazil.";

Должно быть так:
"Acai (Euterpe olereacea), pronounced (ah-sah-ee) is a fruit that grows on the Acai Palm Trees in the Amazonian Rainforest of Brazil.";

С регулярками можно гибче, а по простому:

$lookingFor = array('(ah-sah-ee)' => '<b>(ah-sah-ee)</b>');
$text = "Acai (Euterpe olereacea), pronounced (ah-sah-ee) is a fruit that grows on the Acai Palm Trees in the Amazonian Rainforest of Brazil.";

echo strtr($text , $lookingFor);