I've tried this with several very short testing scripts and get no profiling data for the last function called, whether it is user defined or a built-in function. Example, I have:

$names = array("Tom", "Theresa", "John", "Bill", "Mahatma", "Zedidiah");
$name_string = implode(", ", $names);
$x = strlen($name_string);

which outputs:

0.0004069805 1 *{main}() g:\path.php:0
0.0000200272 1 implode() g:\path.php:161

if I change it to
$names = array("Tom", "Theresa", "John", "Bill", "Mahatma", "Zedidiah");
$name_string = implode(", ", $names);
$x = strlen($name_string);
print_r($x);

it outputs:

0.0003850460 1 *{main}() g:\path.php:0
0.0000188351 1 implode() g:\path.php:161
0.0000019073 1 strlen() g:\path.php:162

In all cases, the last function called is omitted from profiling data.