Analog Input. In this example we use a variable resistor (a potentiometer or a photoresistor), we read its value using one analog input of an Arduino or Genuino board and we change the blink rate of the built-in LED accordingly. The resistor's analog value is read as a voltage because this is how the analog inputs work.
For the Haigis formula, the a0 constant moves the power prediction curve up, or down, in much the same way that the SRK/T A-constant. The a1 constant is tied to the measured anterior chamber depth and the a2 constant is tied to the measured axial length.
PHP 5 Constants. Constants are like variables except that once they are defined they cannot be changed or undefined. PHP Constants. A constant is an identifier (name) for a simple value. The value cannot be changed during the script. A valid constant name starts with a letter or underscore (no $ sign before the constant name). Note: Unlike variables, constants are automatically global across the entire script. Create a PHP Constant.