PHP extension: when values in persistent_list are destroyed?

Question

Our distributed database have php client which is developed on php extension. We store our database object in persistant_list.

Problem: From the log we find, for the same process, sometimes it can't find the database object from persistent_list, have to init the db object in persistant_list, but later(probably 1s)it can't find the same key-value again. It seems the value in persistent list is destroyed. Based on my poor knowledge about php, the values in persistent_list only destroyed by zend_hash_del or web server down. Source code:

if (zend_hash_find(&EG(persistent_list), hash_key, hash_key_len+1, (void **) &le) == FAILURE) {
    tc = tair_init();
    last_rst = tair_startup(tc,uri);
    if(last_rst != TAIR_RETURN_SUCCESS){
        return -1;
    }

    zend_rsrc_list_entry new_le;
    new_le.type = le_tair;
    new_le.ptr  = tc;

    /* register new persistent connection */
    if (zend_hash_update(&EG(persistent_list), hash_key, hash_key_len+1, (void *) &new_le, sizeof(zend_rsrc_list_entry), NULL) == FAILURE) {
        tair_deinit(tc);
        tc = NULL;
    } else {
        rsrc_id = zend_list_insert(tc,le_tair);
    }
}else if (le->type != le_tair || le->ptr == NULL) {
    zend_hash_del(&EG(persistent_list), hash_key, hash_key_len+1);

    tc = tair_init();
    last_rst = tair_startup(tc,uri);
    if(last_rst != TAIR_RETURN_SUCCESS){
        return -1;
    }

    zend_rsrc_list_entry new_le;
    new_le.type = le_tair;
    new_le.ptr  = tc;
    if (zend_hash_update(&EG(persistent_list), hash_key, hash_key_len+1, (void *) &new_le, sizeof(zend_rsrc_list_entry), NULL) == FAILURE) {
        tair_deinit(tc);
        tc = NULL;
    } else {
        rsrc_id = zend_list_insert(tc,le_tair);
    }
}else {
    tc = (tair_handler)le->ptr;
    rsrc_id = zend_list_insert(tc,le_tair);
}

PHP_MINIT_FUNCTION(tair)
{
    REGISTER_INI_ENTRIES();
    tair_set_loglevel(tair_globals.log_level);
    le_tair = zend_register_list_destructors_ex(NULL,tair_dtor,"Tair session", module_number);
    return SUCCESS;
}

Can anyone tell me what's wrong with my php zend engine? Btw the client side use Nginx+fpm.