Description:
------------
pg_query_params(): NULL converts to empty string, but only after looping over the params array with a reference.

php version: 7.0.5-1~dotdeb+8.1
tested against postgres 9.5

Test script:
---------------
$sql = "INSERT INTO params_test (value, details) VALUES ($1, $2)";

$params = array(null, "insert before looping with a reference");
$result = pg_query_params($this->connection, $sql, $params);

$params2 = array(null, "insert after looping with a reference");
foreach ($params2 as &$p) {
    // doing nothing
}
unset($p);
$result = pg_query_params($this->connection, $sql, $params2);


Expected result:
----------------
I would expect that both calls to pg_query_params() would insert SQL NULL as 'value'.

Actual result:
--------------
The first call to pg_query_params() inserts SQL NULL as expected:
  2016-04-15 11:59:07 EEST LOG:  execute <unnamed>: INSERT INTO params_test (value, details) VALUES ($1, $2)
  2016-04-15 11:59:07 EEST DETAIL:  parameters: $1 = NULL, $2 = 'insert before looping with a reference'

The second call inserts empty string as 'value' which is unexpected:
  2016-04-15 11:59:07 EEST LOG:  execute <unnamed>: INSERT INTO params_test (value, details) VALUES ($1, $2)
  2016-04-15 11:59:07 EEST DETAIL:  parameters: $1 = '', $2 = 'insert after looping with a reference'