Sample code:
<?php
function myfunc ($arg) {
return trim ($arg);
}

function funcall_pre_cb() {
print "funcall_pre_cb\n";
}

function funcall_post_cb() {
print "funcall_post_cb\n";
}

fc_add_pre ('trim','funcall_pre_cb');
fc_add_post ('trim','funcall_post_cb');

fc_add_pre ('myfunc','funcall_pre_cb');
fc_add_post ('myfunc','funcall_post_cb');

print "--- trim test ---\n";
print trim (' abc ')."\n";

print "--- myfunc test ---\n";
print myfunc (' abc ')."\n";
?>

Result with Xdebug:
--- trim test ---
abc
--- myfunc test ---
funcall_pre_cb
funcall_post_cb
abc

Result without Xdebug:
--- trim test ---
funcall_pre_cb
funcall_post_cb
abc
--- myfunc test ---
funcall_pre_cb
funcall_pre_cb
funcall_post_cb
funcall_post_cb
abc