Where to place Arrays and else issue

Question 1

Your code was missing several braces, also you your $TestValues is part of the class, not a global variable.

I fixed the code, however you have to check the math and read the oop tutorial on php's site.

<?php
class Primes {
    public $TestValues = array(0, 1, 2, 3, 4);
    function IsPrime($number) {

        //if number is a number, perform the rest of the tests, else, return -1 (error)
        if(is_numeric($number)) {

            if($number < 0 || $number > 1024)
                return -1;
            if($number === 0 || $number === 1 || $number % 2 === 0)
                return 0;

            //if number has passed all previous tests, mod it by all odd numbers from 3 to its square root rounded up.
            for($i = 3; $i <= ceil(sqrt($number)); $i = $i + 2) {
                //if any numbers mod 3 to its square root equal 0, return 0, (false, not prime)
                if($number % $i == 0)
                    return 0;
            }
            //if the number has passed all above requirements, then it is a prime number below 1024.
            return 1;
         } else {
            return -1;
        }
    }

    function test() {
        foreach($this->TestValues as $value) {
            $t = $this->IsPrime($value);
            if($t === 0) {
                echo($value . "=> Is not Prime");
            } elseif($t === 1) {   
                echo($value . "=> Is Prime");
            } elseif(IsPrime() == -1) {
                echo($value . "=> Is an Error");
            }
            echo "\n";
        }
    }
}

function main() {
    $prime = new Primes();
    $prime->test();
}

main();

Question 2

You have some syntax errors in your code. I made some modification to your code now it is working :

<?php

// Check the number to be prime or not
function IsPrime($number) 
{
    // If the variable is not numeric, negative or greater than 1024, exit
    if (!is_numeric($number) || $number < 0 || $number > 1024)
    {
    return -1;
    }
    // Perform the tests
    switch($number)
    {
    //if number is 0 or 1, then return 0 (false, not prime)
        case 0 :
        case 1 :
            return 0;
        break;
    //if number is 2, return 1 (true, is prime)
        case 2 :
            return 1;
        break;
    }

    //if number mod 2 is 0, then it is even, and no even number is prime except 2, return 0 (false, not prime)
    if($number % 2 == 0) 
    { 
    return 0;
    }

    //if number has passed all previous tests, mod it by all odd numbers from 3 to its square root rounded up.
    for($i = 3; $i <= ceil(sqrt($number)); $i = $i +2)
    {
        if($number % $i == 0) 
        {
            return 0;
        }
    }

    //if the number has passed all above requirements, then it is a prime number below 1024.
    return 1;
} // End of isprime function

// create an array of numbers
$testvalues = range(-2, 100);

foreach($testvalues as $value) 
{
    switch(isprime($value))
    {
        case 0 :
            echo("<p style='color:gainsboro;'>" . $value . "=> Is not Prime</p>");
            break;
        case 1 :
            echo("<p style='color:green;'>" . $value . "=> Is Prime</p>");
            break;
        case -1 :
            echo("<p style='color:red;'>" . $value . "=> Is an Error</p>");
            break;
    }
}
?>

Notes :

I think you don't need a class here. It is a simple program and using classes will make it a bit complex and php is known for its simplicity. So keep it simple.
In some cases switch is better and more readable than if, so use it if possible http://php.net/manual/en/control-structures.switch.php
Every language has it preferred syntax. In php it is common not to use capitals in naming variable (except for classes) unlike other languages
there is no main() in php

Hope this helps, excuse my english and correct me if I am wrong

Question 3

The reason why u getting error for else statement is because of the closing braces } of your if statement, notice that you have a closing brace after your else condition,instead of doing that, you have to close if statement first then do else statement which in your case else means if is not numeric then do sth, also you have so many if statements which looks a little bit redundant. what you could do is to put them in same condition like if($number ==0 || $number == 2 || $number % 2 ==0 ) return 0 elseif // do sth

The reason why you can't get $Testvalues is that you need to pass the array to your function then you can use it, where did u call your TestIsPrime function? when you call it, do sth like TestIsPrime($TestValues);

btw, it's better not to use Capital letter for variable...