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How to get the values of session using js

I want to know if the user has logged in or not and then display different elements of the navigation based on the username (login by email and password Not username). However, I have no idea how to deal with session. If php and html is separated in two documents, how can I store the required values using session in php document and then get them using javascript in html document? Or should I use cookies instead?

There are a several approaches to do this.
1) You can make a PHP file which will format your $_SESSION data, and all the other data you want as a JSON string (json_encode function in PHP lang). Then use echo to return it. Then use an AJAX request to get this file from javascript or JQuery, and you will receive the data you want. That's a bad approach for this purpose, from my point of view, because after loading the page you send another request to receive a static data, which will not change on the page like email or username.
2) Better approach: PHP is preprocessor hypertext. You can make an php file and write html tags in it, with php output(example: <div><?=$_SESSION['email']?></div>). Search for more info in google ("php inside html").
3) Much better. In modern web programming world its a mistake to use php inside html because you should think not only about how to make something work, you should think how you will maintain it after 3, 6, 12 months later, too. If you use just php inside html in a big project, then, with time, you realize that your code is hard to read and looks ugly. There are plugins that can make your view more readable and maintainable (Twig, Blade, Volt and others). I recommend you use one of them.

The session is a server side thing, you cannot access it using javascript. You can write an Http handler (that will share the sessionid if any) and return the value from there using AJAX

How do I populate form fields on a page from a database based on the page url?

Hello first I’d like to say, please excuse my ignorance to this all, as I’m very new to this all. I just started and still trying to understand this.
So far I have a database set up and I’m trying to retrieve values from a database to fill in a form on a page when it loads. The record or row/values that need to be retrieved from a database depend on the page’s URL.
I’m ok with html and css but still trying to learn more about jquery, JavaScript, sql, php and so on and so forth. I realize I have a ways to go and honestly some of the guides and tutorials online are kind of confusing because everyone has a different way of coding. So I’m a bit confused.
I’ve included a simple chart to breakdown what I’m trying to do.
If someone could point me in the right direction I’d be really grateful! Thanks.

If I understand you well, you want to setup a form and populate some fields of this form with a query forms a database, the primary key of the record being relative to the url.
The first step is to build the url, you can pass some parameters to an url by adding a ? at the end of the url followed by the parameter name, the = sign and the parameter value. If you have more than one parameter, you should separate each parameter with the sign &.
So your url could be something like this :
www.examplesite.com/page&.html?key=key_A1
Then, you'll have to choose if you want to build the page on the server side with a language like php, in which case you retrieve the parameter, query the database, build the html form and send it to the client.
You can also go client side with plain javascript or jquery, in which case you will still have to do some server side programming to query the database but will use an ajax call to get the data and will populate the fields in Javascript or JQuery.

You can do this using Javscript(using Ajax)
U need the key to search for get the results from database
Using ajax call get the data, You might have to write code to get the details from database using any server side prog language
Using javascript to fill the form input with the received data.
Google for jquery ajax examples, and how to populate input using javascript.
There are many frameworks out there that have DataBinding built-in that do the same job a lot easier. My favourite would be Angular Js, you can try Ember and lot more out there, choose the one you feel comfortable with.

How to write script for table filtering

I'm fairly new in web design/code and I'm trying to figure out/learn how to write script that would execute my table filter. As I found out this would be easiest to do in PHP since all data comes from table itself (and not MySQL DB).
Here is example of HTML with test table and some filters I would like to implement, but I've been having problem with writing PHP script which would work properly.
http://www.bonemachineonline.com/test
(I uploaded example on my own site)
I would appreciate if someone could help me out with this in anyway! Writing example or explaining how/what to do. Anything is more than appreciate!

Try using jquery dataTable().
You can refer the below link
Jquery-dataTable()

You have 3 approaches for data filtering.
If the table is filtered by default, and all the data are not loaded, take the (1) or (2).
If you load the whole table each time, take the (3).
Approaches
(1) Server side, with page reloading : your user checks filters, then clicks on a submit button. You get all the checked filters and you send it to your PHP. Server side, you retrieve the filters and remake your query (SQL or other in your case).
(2) Server side, with AJAX : exactly the same mechanism, but you send the filters to your PHP with an AJAX request, wait a response, remove your table and remake a table with the new data.
(3) Client side, with JavaScript only : when your user checks a filter, you trigger the change event and hide the matched elements in the table.
Some tips
For the approaches (2) and (3), jQuery could be very useful.
For the approach (3), you can use a library. Here: 35+ Useful jQuery Filter and Sort Plugins.
For the approach (1), if in your case the data are in a PHP array, you have a lot of functions to manipulate this array in function of your filters. Here: Array Functions - Table of Contents.

Drop down Selection -> Search database

How would I search an SQL database when a user makes a selection in a dropdown list. I know how to change/add text to areas using some javascript but not with a SQL search as well.
Ideally I don't want to be changing pages in this process as I'm thinking they won't be sticking on a single option in the drop down for long.
Cheers in advance. =)

You have to add an onchange event listener to the dropdown box. When the user selects an option, an AJAX (XmlHttpRequest) should be send to the server. A serverside script, a PHP page for example, should parse the parameters (after checking).
When these parameters have been checked, they should be escaped for this reason. Perform a search query, parse the results, and send the output back to the client (the user, browser).
Useful links:
http://www.php.net/manual/en/ref.mysql.php
https://developer.mozilla.org/en/xmlhttprequest

you should look into implementing Ajax for this.
Here's a simple example and tutorial utilizing a drop down list that fetches information from a database on selection, without changing pages :)
http://www.w3schools.com/php/php_ajax_database.asp
You should also look into using jquery for your Ajax requests as stated in the answer below. (Do more with less code).
See also:
http://15daysofjquery.com/quick-and-dirty-ajax/14/ for a simple jquery Ajax tutorial.

Before you render the page, make sure you do a grab the info you want presented from the database.
select distinct NAME, ID
from tableName
You can use the information obtained here to generate the html for a drop down box. Include something to the effect
<select onchange="doAction(this.value);">
<option value="userID">userName</option>
<option value="userID">userName</option>
<option value="userID">userName</option>
...
</select>
Then perform an AJAX request back to your site. Get your information from your second query to the database, and return with the information you need. When you perform your second query (and in general) you have to protect yourself against injection attack vectors. The best start for this is to make sure you have encoded(HTML, SQL) correctly.

Without more information I can only really give you the below as a guide. It uses the jQuery load function to put the contents of one page into an element. Note that .load cannot be used across domains.
$("#searchButton").live("click", function() {
$("#results").load("/SearchResults.php?s=" + $("#searchBox").val());
});
Also note that you should use something to filter out HTML tags to prevent SQL injection, as well as taking other measures.

Obtaining value obtained by javascript using PHP

I was able to extract user information from the social Graph API and display it on my site within tags . How do I now get this information, and use it in a PHP script? In other words, say I want to use PHP to calculate the weather, but only based on the preferred weather on a users facebook profile(hypothetical situation). Now, assuming I have obtained the users preferred weather using javascript, how do I use PHP to get that information in order to perform my calculations? I guess one way would be to use regular expressions to match the values on the same page, but that seems tedious. Any help/direction is appreciated.

Are you using php to access the Facebook API or the pre built HTML they provide? You should use php to retrieve the data, do what you need to do with the data, and then display it on a webpage, rather than the other way around.

You can use javascript to get the data, then send it to your php script using AJAX, but this is a round-about way of doing it.
You can make the call the to the Graph API directly from your PHP script. Something like this:
$r = new HttpRequest('https://socialgraph.googleapis.com/<method_name>', HttpRequest::METH_GET);
You will need to parse the data when you get the response -- json_decode() should do the trick.