Description:
------------
I try to use persistent connection to database with tls protocol. Also i use nginx and php-fpm. During first 6 requests connections to db are established. In next request I receive php Warning:
Errno: 2 
Errstr: oci_parse(): supplied resource is not a valid oci8 connection resource 
Errline: 16

If I restart php-fpm, then i can do 6 requests more, and in 7th I will recive warning.
If in php-fpm I set pm.start_servers = 10 and restart php-fpm, then i can do 11 requests, and in 12th I will recive warning.
Requests without error correlates with pm.start_servers parameter.

Also, in cloned vm with php version 5.6.22 problem does not exists.
Also, oci_pconnect() with php7.1.3 works without tls protocol.


OCI8 Version 2.1.3
Oracle Run-time Client Library Version 12.2.0.1.0
Oracle Compile-time Instant Client Version 12.2
php.ini default settings

php.fpm settings:
pm.max_children = 250
pm.start_servers = 5
pm.min_spare_servers = 5
pm.max_spare_servers = 25

Test script:
---------------
<?php

ini_set('error_reporting', E_ALL);  
ini_set('display_errors', 1);  
function errorsHandler($errno , $errstr, $errfile, $errline) {  
    echo "Errno: " . $errno . "\n";  
    echo "Errstr: " . $errstr . "\n";  
    echo "Errline: " . $errline . "\n";
    die();  
}  
set_error_handler('errorsHandler');
$conn = oci_pconnect('/', '', 'SECUREDCONNECTION','AL32UTF8',OCI_CRED_EXT);  
if (!$conn) {  
  die("Connection problems: " . oci_error());  
}  
$stid = oci_parse($conn, 'select * from accounts where number = :number');  
$value = '4080************2622';  
oci_bind_by_name($stid, ':number', $value, -1, SQLT_CHR);  
oci_execute($stid);  
echo "Sql executed" . PHP_EOL;  
oci_free_statement($stid);

Expected result:
----------------
Sql executed

Actual result:
--------------
Errno: 2 
Errstr: oci_parse(): supplied resource is not a valid oci8 connection resource 
Errline: 16