PHP Simple AST example

fiddle_603623.php

<?php

function evaluate($env, $expr) {
	list($e, $v) = each($expr);	
	
	switch(strtolower($e)) {
		case "add":
			return evaluate($env, array_slice($v,0, 1)) + evaluate($env, array_slice($v, 1, 1));
		break;
		case "multiply":
			return evaluate($env, array_slice($v,0, 1)) * evaluate($env, array_slice($v, 1, 1));
		break;
		case "variable":
			return $env[$v];
		break;
		case "number":
			return $v;
		break;
	}
}

$environment = array("a" => 3, "b" => 4, "c" => 7);

$expression_tree = array(
   "add" => array(
       "variable" => "a",
       "multiply" => array(
		   "number" => 2,
		   "variable" => "b"
	   )
	)
);
	
	
echo evaluate($environment, $expression_tree);


function number($n) {
	return function($env) use ($n) {
		return $n;
	};
}
function variable($a) {
	return function($env) use ($a) {
		return $env[$a];
	};
}
function add($a, $b) {
	return function($env) use ($a, $b) {
		return $a($env) + $b($env);
	};
}
function mul($a, $b) {
	return function($env) use ($a, $b) {
		return $a($env) * $b($env);
	};
}

function evaluate2($tree, $env) {
	return $tree($env);
}

$tree = add(variable("a"), mul(number(2), variable("b")));
echo evaluate2($tree, $environment);			
?>