$test = 'This is a text with an $id inside';

$matches = preg_split('#(\$[a-z]*)#', $test,
PREG_SPLIT_DELIM_CAPTURE);

echo '<pre>';
print_r($matches);
echo '</pre>';

expected output:
Array
(
    [0] => "This is a text with an "
    [1] => $id
    [2] => " inside"
)

output produced in the real world:
Array
(
    [0] => "This is a text with an "
    [1] => " inside"
)

maybe this will help to hunt the bug down.
-- edit
important note: actually no bug at all, missed the limit param.
---
a wise man said: do not report a bug after a hard 10 hour work day.