Variable

A variable is a placeholder for a value within an expression. A valid variable name conforms to PHP's variable notation, excluding the prefixed dollar sign:

A valid variable name starts with a letter or underscore, followed by any number of letters, numbers, or underscores.

A variable must be assigned either an int or a float type value. Unlike constants, a variable may be defined in an expression without any registration beforehand. As variables follow the exact notation as constants, it is not possible to define pi, for example, as a variable.

$expression = $parser->parse("x + pi");
$expression->evaluate(["x" => 2, "pi" => 1]); // 5.1415926535898, not 3

Expression::findVariables() lists out variables occurring within an expression.

$expression = $parser->parse("x * x + z * z");
$variables = iterator_to_array($expression->findVariables()); // ["x", "x", "z", "z"]