<?php
$companies = array(
'apple' => 'AAPL',
'baxter' => 'BAX'
);
$tweet = <<< TXT
this is bapple and apples
TXT;
foreach ($companies as $name => $ticker) {
$tweet = preg_replace('/\b'.preg_quote($name).'s{0,1}\b/', "<b>{COMPANY|$ticker}</b>", $tweet);
}
var_dump($tweet);