You need to declare variables before echoing them out. An example is here:

<?php
    $var = "test";
    echo $var; // it will echo out test
?>

And trying to echo out a variable this way will generate an error:

<?php
    echo $var; // it will generate error
    $var = "test";
?>

In addition, you can declare variables in another file and can include that file to echo out the variable somewhere. Remember to include the file first and then call it.

Example vars.php:

<?php
    // define vars
    $var1 = "Test 1";
    $var2 = "Test 2";
?>

Now in another file, include vars.php first and then call the variable:

<?php
    require_once"vars.php";
    echo $var1;
?>