The version of PHP that this bug was reported in is too old. 
Please try the php-4.2.0RC3 from http://www.php.net/~derick/ 

If you are still able to reproduce the bug, please change the PHP
version on this bug report to the version you tested and change 
the status back to "Open".

This bug should be fixed now.
You were right, the problem was in ibase_close().
However have a look at:
http://www.php.net/manual/en/function.ibase-close.php

Daniela

Apparently, it has not been reported that this problem is now again in version 7.0 (at least, I have not checked 7.1).

For 3 years this statement:

while($fl = ibase_fetch_object($res)) {
 // $res = sql with 17 columns and a maximum of 30 rows
}

had been working seamlessly in up to version 5.6. As the app is now in a server with php 7.0, this is the only statement failing (so far).

this is broken code anyways, a sane IDE would tell you (assignment in condition)
while($fl = ibase_fetch_object($res))

the docs don't tell anything at http://php.net/manual/de/function.ibase-fetch-object.php but when you look at http://php.net/manual/de/mysqli-result.fetch-object.php it should look like

while(($fl = ibase_fetch_object($res)) !== NULL)

I discovered where the problem was.

I versions prior to 7.0, it was possible to execute

ibase_connect( ... ) a second (or third time or more) without anything like this happening.

In version 7.0, if a function tries to execute ibase_connect() then the environment from where such function was called loses values of variables and can not continue.

As the function called, which executed a second ibase_connect in one POST, returns, then the previous environment breaks.

I had this:

while(($fl = ibase_fetch_object($res)) !== NULL) {
  glGetThat();
}

function glGetThat() had a connection, which would be redundant, but after it returned from glGetThat() then $res had lost the value it had to continue retrieving the query results.

I hope this is useful, because a non-documented change has happened in version 7.0 which affects the way ibase_connect() behaves.

See also Bug #72175