I need to control an external page designed and developed in php that takes values from an external database. What I actually need is to edit and change some hardcoded content of the page with wordpress. How can this be done?? Any sugestions?
Assuming both are parts of same site you can load WP engine in that page and use WordPress functionality in it, see Integrating WordPress with Your Website.
But it would probably make more sense to convert that page into WP page template or something. Hard to say without knowing specifics.
I do not think you can use the existing post and page editing code for external content. I will suggest you try to develop a plug-in instead that will query the custom page content and display it for editing. You should be able to reuse the TinyMCE editor as it is already included.
I would also recommend that you consider using the Pod CMS plugin instead; it is a generic CMS plugin system integrated with WordPress administration backend, and is powerful enough for complex data manipulation and may meet your needs.
I'm not sure I totally understand your question - I think you're asking how to include the content of a WordPress "page" on a page displaying mainly externally-stored content. The way I've handled this in the past has been to set up additional template files in your theme, one per necessary page type, that just include the $content from the WP database in the right place among the externally-stored content.