Hi @Shopi ,
If you want to embed your app into the Shopify Admin, you need to utilize the Shopify App Bridge. You can read more about this here: https://shopify.dev/tools/app-bridge
If you decided to build your app in Ruby instead of PHP, you can also use the shopify_app library to build an embedded app (https://github.com/Shopify/shopify_app) or if you are using Node JS you can use the koa-shopify-auth middleware (https://github.com/Shopify/quilt/tree/master/packages/koa-shopify-auth) instead