A variable quantity is a quantity that can change or vary, such as the temperature, time, or the price of a product. These quantities can take on different values and are often represented by symbols or letters in mathematical equations. On the other hand, a non-variable quantity is a quantity that remains constant and does not change, such as the speed of light or the number of sides in a triangle. Non-variable quantities are fixed and do not depend on any other factors.

Variable definition is the process of declaring a variable and specifying its data type, such as int, float, or string. This tells the compiler or interpreter that a variable with a certain name and data type will be used in the program. Variable initialization, on the other hand, is the process of assigning a value to the variable for the first time. This can happen at the same time as the variable is defined, or at a later point in the program. Initialization gives the variable a specific value to work with, while definition simply sets up the variable's characteristics.

In Arduino, you can write a variable in a variable by using the concept of pointers. You can declare a pointer variable and then assign the address of the original variable to the pointer variable. This allows you to indirectly access the original variable through the pointer variable. Here's an example of how to write a variable in a variable in Arduino: ```C int originalVariable = 10; int *pointerVariable = &originalVariable; // Assign the address of originalVariable to pointerVariable *pointerVariable = 20; // Write a new value to originalVariable through pointerVariable ``` In this example, the value of originalVariable is changed to 20 by writing to it through the pointerVariable.

One can express a variable in terms of another variable by manipulating equations or formulas to isolate the desired variable on one side of the equation. This can involve performing algebraic operations such as addition, subtraction, multiplication, or division to rearrange the equation. By doing so, the variable of interest can be written in terms of the other variable, allowing for a clearer understanding of the relationship between the two variables.

To store a JavaScript variable in a PHP variable, you can use AJAX to send the JavaScript variable to a PHP script on the server. The PHP script can then receive the variable using $_POST or $_GET superglobals and store it in a PHP variable. This way, you can pass data between JavaScript and PHP seamlessly.

A JavaScript variable can be transferred to a PHP variable by using AJAX (Asynchronous JavaScript and XML) to send the variable value to a PHP script on the server. The PHP script can then receive the variable value using the $_POST or $_GET superglobals, and assign it to a PHP variable. Another way to transfer a JavaScript variable to a PHP variable is by embedding the JavaScript variable value in a form and submitting the form to a PHP script, which can then retrieve the value using the $_POST or $_GET superglobals.

You can output a batch variable in a variable name by using the delayed expansion feature in batch scripting. To do this, you need to enable delayed expansion by using the "setlocal enabledelayedexpansion" command at the beginning of your script. Then, you can use the "!" symbol instead of "%" to access the value of a variable inside another variable. For example, if you have a variable named "var1" and you want to output its value in a variable named "var2", you can use the syntax "!var1!" to achieve this.

Variable declaration refers to the process of defining a variable in a program, including its name and data type. This tells the program that a certain variable exists and can be used. Variable initialization, on the other hand, refers to the process of assigning a value to a variable at the time of declaration or later in the program. This gives the variable an initial value to work with. In summary, declaration is about creating the variable, while initialization is about giving it an initial value.