To start development using QB API, you need to create an IA apps in Intuit's appcenter. https://developer.intuit.com/docs/0025_quickbooksapi/0010_getting_started/0010_signup
From the above link you will get - apptoken, consumer key and consumer Secret. You can use the above 3 keys in IPPOAuthPlayground(PFB link) to get the access token and access secret corresponding to your QB Online account. https://appcenter.intuit.com/Playground/OAuth
Intuit provides PHP devkit which you can download from the link below. You can simply plugin the above OAuth tokens in devkit and call any QB endpoints.
Download Link - https://developer.intuit.com/docs/0025_quickbooksapi/0055_devkits
User Guide ( code snippets ) - https://developer.intuit.com/docs/0025_quickbooksapi/0055_devkits/0210_ipp_php_sdk_for_quickbooks_v3/0002_synchronous_calls/0001_data_service_apis
Thanks