PHP as mentioned earlier was originally designed in the year 1994 but released in 1995. It is a language script that is exclusively used for designing very interesting webpages. It is commonly recognized as software that is free to download as an open source type. In the files it is executed using the php runtime to create dynamic pages. The functions of php can be used in any web browsers or servers and it can work on different stages like windows, UNIX, Linux, etc. Additionally, the script can also be used with numerous relational database management systems (RDBMS).
Although initially it was developed to create vibrant webpages, now the functions of php centers on SSS (server side scripting). This is where the script supplies contents of a server to the customer. It is not unusual to find php files containing scripts and html tags. It is treated by the website server and sent back to the browser as a simple html. While HTML is identified as "HTML," php files are determined by "php." Another interesting thing about it is that it is easy to create especially if you follow php guides through passing through php tutorials.
There isn't much difference between the two, but the major one is that while HTML is use as a mark up language to identify a website content, php is identified basically as a scripting language. Webpages are designed using only html are static, pages designed using php are dynamic.
These are basically the php vs. html or the difference between php and html. The differences are not much.