I revoke the statement of SIGCHLD, it happens even without the handler in some situations, the behaviour seems erratic. Perhaps the database connection is shared with the children, and when the child dies the db variable the connection is closed. 

I tried closing the DB connection first thing on the child but with no results, the error still appears when the child dies.

When using persistent connections, even if reopening a new connection *after* forking, the connection fails. The only way to have DB connections is to reopen them after forking, with PDO::ATTR_PERSISTENT => false

Can you provide a test script for your issue?

Closing as no feedback has been provided since I asked for it in 2012.

I confirm the existence of this error in php 7.

Here is example code:

test1.php:
<?php
defined('APPLICATION_PATH')
|| define('APPLICATION_PATH', realpath(__DIR__ . '/application'));
defined('APPLICATION_ENV')
|| define('APPLICATION_ENV', (getenv('APPLICATION_ENV') ? getenv('APPLICATION_ENV') : 'development'));
require_once 'Zend/Application.php';
$application = new Zend_Application(
    APPLICATION_ENV,
    APPLICATION_PATH . '/configs/application.ini'
);

$db = new Zend_Db_Adapter_Pdo_Pgsql(array(
    'host' => 'localhost',
    'username' => 'test',
    'password' => 'hasłodlatest',
    'dbname' => 'test'
));

echo 'start' . "\n";
$select = $db->select()->from('aaa', '*');
print_r($db->fetchOne($select));
echo "\n";

$pid = pcntl_fork();
if ($pid == -1) {
    // pcntl_fork() failed
    echo('could not fork');
} elseif ($pid) {

} elseif ($pid == 0) {
    // you're in the new (child) process
    exec('/usr/bin/php /home/idea/CLI/test2.php &>/home/idea/CLI/test.log &');
    exit;
}

echo 'After fork' . "\n";

while (true) {
    sleep(5);
    $select = $db->select()->from('aaa', '*');
    print_r($db->fetchOne($select));
}

test2.php:
<?php
echo 'Forked' . "\n";