Note before: I have no idea what level of knowledge you have with this stuff some I'm start from the top, sorry if I seem undermining.
OK, first the <script> tag. This tag lets you declare a sub section of your HTML code in which you can use another language. HTML is rather static on its own and so other languages like JavaScript give you the ability to insert some more dynamic and interactive code.
Another language you could use in the <script> tag is VBScript (Visual Basic Script) however this is limited to Windows clients as Visual Basic is a Microsoft language for scripting elements of the Windows environment which can be inserted into a HTML page or HTA (HTML Application). A slightly inaccurate parody would be if you could insert bash script into you bash for the clients browser to interpret along side the HTML. Visual Basic Script, VBScript or VBS is a variation on a full blown high level application language called Visual Basic which is a for writing high level applications within Windows (again another slightly inaccurate parody would be to imagine if you could insert 'PythonScript' directly into your HTML pages giving you a less powerful version of python to work with.
On the to PHP stuff. PHP is a server side language. What does this mean? It means than you can write a HTML page in your favourite text editor and then open it in your favourite browser and it will interpret the result and display a page. You could even throw some JavaScript in there (don't confuse JavaScript with Java!). This can all be done on your desktop.
PHP is a server side language which means you need a web service (like Apache) running serving up your pages like a web server with the PHP extensions/interpreter installed. What this means is that you can write a page in HTML and throw some PHP in. Then when a client browses to mywebserver.com/my_php_page.php on the server, instead of simply parsing the client browser the code of the page to render the server sees that the file has a .php extension and reads and executes the PHP code sending the resulting HTML code of that PHP code execution to the clients browser.
HTH and didn't confuse you more!