Yes Zen Cart has a lot of difference from osCommerce even if it is based on osCommerce.

Front Page Flow

1. Load application_top.php
1.1 load local configure file (/includes/local/configure.php)
1.2 set the level of error reporting
1.3 include server parameters (/includes/configure.php)
1.4 include the extra configure files in /includes/extra_configures/
1.5 load the autoloader files in /includes/auto_loaders/
1.6 load the autoloader interpreter code (/includes/autoload_func.php)
init_file_db_names
init_database
include version.php
init zc_cache object
init_db_config_read : read the configuration settings from the db
create sniffer object
init_gzip
init_script
create phpBB object
init_general_funcs
init_tlds
init_sessions
create cart session object
create navigation session object
create curriences object
init_sanitize
init_languages
init_templates
navigation.add_current_page
init_currencies
create messageStack object
init_customer_auth
init_cart_handler
main_cart_actions
init_special_funcs
create breadcrumb object
init_category_path
init_add_crumbs
init_header
1.7 load the counter code (/includes/counter.php)
1.8 get customers unique IP
2. Set main language directory based on $_SESSION['language']
3. Load all *header_php.php files from includes/modules/pages/PAGE_NAME/
4. Load html_header.php (a common template file)
5. Load main_template_vars.php (a common template file)
6. Load on_load scripts (page based and site wide)
7. Load tpl_main_page.php (a common template file)
8. Load application_bottom.php


Admin Page Flow

1. Load application_top.php
1.1 load local(user created) configure file (includes/local/configure.php)
1.2 load the main configure file (includes/configure.php)
1.3 ignore version-check if INI file setting has been set
1.4 include extra configure files (includes/extra_configures/*)
1.5 load the default application_top autoloader file
1.6 load the autoloader interpreter code
create notifier object
init_file_db_names
init_database
init_db_config_read
init_gzip
create sniffer object
init_general_funcs
init_tlds
init_sessions
init_languages
init_templates
create products object
create messageStack object
init_special_funcs
init_category_path
init_errors
init_admin_auth
init_admin_history
init_html_editor
2. Load header.php
3. Page process
4. Load footer.php
5. Load application_bottom.php