I have a class with a required file:

require_once 'includes/constants.php';

That required file contains the following:

<?php

// Define constants here

define("DB_SERVER", "localhost");
define('DB_USER', 'xxxxxxxxxx');
define('DB_PASSWORD', 'xxxxxxxxxx');
define("DB_NAME", "customerInfo");
define("ERROR_FILE", "c:\wamp\www\customerinfo\Errors.txt");

The class has a constructor:

    function __construct() {
        $dbServer = DB_SERVER;
        $dbName = DB_NAME;
        $dbUser = DB_USER;
        $dbPWD = DB_PASSWORD;
        $errorFile = ERROR_FILE;
        echo "Error file in mysql construct: " . $errorFile . '<br />';
        //$dsn = DSN;
        try {
            $this->conn = new mysqli("$dbServer", "$dbUser", "$dbPWD", "$dbName") or die();
            //$this->conn = new PDO($dsn);
            //$this->conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_WARNING);
        }
        catch (Exception $e) {
            echo "There has been a problem connecting to the database.\nPlease refer to $errorFile for the cause of the problem.\n\n";
            file_put_contents($errorFile, $e->getMessage(), FILE_APPEND);
        }

One of the functions/methods in the class is this (I put it in for debugging):

    function ShowErrorFile() {
        echo "Error File in ShowErrorFile function: " . $this->errorFile . '<br />';
    }

When I call that method from another php page, like this:

                    <?php
                        $mysql = New Mysql();
                        $mysql->showErrorFile();
                    ?>

The error file comes up empty/null and I don't understand why. The only way I can get the methods within the class to see the error file is by including this at the top of the class:

private $errorFile = ERROR_FILE;

Can someone explain to my dumb brain what's going on? I thought that by including the error file definition in the constructor, the methods would know what the error file is.

When referencing class instance variables, you need to do so as follows:

$this->errorFile = ERROR_FILE;

Did you define $errorFile as a class variable ?

class MySql {
    private $errorFile;

I did define $errorFile as a class variable, like this:

class Mysql {
    private $conn;
    private $dbServer;
    private $dbName;
    private $dbUser;
    private $dbPWD;
    private $errorFile;

    function __construct() {
        $dbServer = DB_SERVER;
        $dbName = DB_NAME;
        $dbUser = DB_USER;
        $dbPWD = DB_PASSWORD;
        $this->errorFile = ERROR_FILE;

What I don't understand yet, though, is that within the construct, if I apply the $this-> construction to the other class variables (conn, dbserver, etc...), then they quit working. Yet the errorFile variable only works if I use the $this-> construction. Any thoughts?

If you omit $this-> it is considered a local variable. It goes out of scope at the end of the constructor. Perhaps you are not referencing them correctly.

When handling class instance variables, you always need to use $this->. Therefore make sure you update every reference, as @pritaeas said. E.g.:

class Mysql {
    private $conn;
    private $dbServer;
    private $dbName;
    private $dbUser;
    private $dbPWD;
    private $errorFile;

    function __construct() {
        $this->dbServer = DB_SERVER;
        $this->dbName = DB_NAME;
        $this->dbUser = DB_USER;
        $this->dbPWD = DB_PASSWORD;
        $this->errorFile = ERROR_FILE;
        echo "Error file in mysql construct: " . $this->errorFile . '<br />';
        //$dsn = DSN;

        try {
            $this->conn = new mysqli($this->dbServer, $this->dbUser, $this->dbPWD, $this->dbName) or die();
            //$this->conn = new PDO($dsn);
            //$this->conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_WARNING);
        }
        catch (Exception $e) {
            echo "There has been a problem connecting to the database.\nPlease refer to $errorFile for the cause of the problem.\n\n";
            file_put_contents($this->errorFile, $e->getMessage(), FILE_APPEND);
        }
    }

Thanks guys, I think you've helped me solve this!