oci_free_statement doesn't always free up cursors.  I had a query where I performed the following functions in a loop:

OCIParse
OCIExecute
Oci_fetch_assoc
(Grab some field values)
OciFreeStatement

I didn't specify the use of a cursor, but ran into a "maximum 
open cursors exceeded" error.  Within my code, I had one "select * from table_with_lobs" query.  When I changed the query to be "select a, b, c, from table_with_lobs" (where I specified the actual column names and where those columns were not LOB fields) the error message went away and I didn't have to resort to upping the max_open_cursors value in Oracle.

If you are using cursors, make sure to free the statement *and* the cursor, especially if there is a possibility of running the proc/cursor again (e.g. with different parameters). 

<?php

    oci_execute($stmt);
    oci_execute($crsr);

    // iterate through cursor...
    
    oci_free_statement($stmt);
    oci_free_statement($crsr);
?>

You need to do it explicitly, closing connection for example does not seem to release the cursor.

A freed statement is not "empty()", it's still a resource:

<?php
$con=oci_connect(/*connect*/);
$q=oci_parse($con,"SELECT sysdate FROM dual");
var_dump($q); // resource(5) of type (oci8 statement)
var_dump(empty($q)); // bool(false)
var_dump(oci_statement_type($q)); // string(6) "SELECT"
echo "------\n";
oci_execute($q);
var_dump($q); // same as above
var_dump(empty($q));
var_dump(oci_statement_type($q));
echo "------\n";
oci_free_statement($q);
var_dump($q); // resource(5) of type (Unknown)
var_dump(empty($q)); // bool(false)
var_dump(oci_statement_type($q)); // generates warning and returns false
oci_close($con);
?>

So far I can not think of a way to determine if a statement is freed without using an additional flag...