lisp in php, based on http://danthedev.com/2015/09/09/lisp-in-your-language/

lispeval.php

<?php

class SpecialForm {
    private $function = null;
    private $callback = null;

    public function __construct($function,$callback) {
        $this->function = $function;
        $this->callback = $callback;
    }

    public function __invoke() {
        $args = func_get_args();
        return call_user_func_array($this->callback,$args);
    }
}


function lispEval($rawExpression,$localScope = []) {
    if (is_null($rawExpression)) {
        return;
    }
    if (is_scalar($rawExpression)) {
        return $rawExpression;
    }    
    static $globalScope = [];

    $scope = array_merge($globalScope,$localScope);
    
    $natives = [
        '+' => function($a,$b) use($localScope) {
            return lispEval($a,$localScope)+lispEval($b,$localScope);
        },
        '-' => function($a,$b) {
            return $a+$b;
        },
        '*' => function($a,$b) {
            return $a*$b;
        },
        '/' => function($a,$b) {
            return $a/$b;
        },
        '=' => function($a,$b) {
            return $a === $b;
        },
        '>' => function($a,$b) {
            return $a > $b;
        },
        '<' => function($a,$b) {
            return $a < $b;
        },        
        '.' => function($a,$b) {
            return $a . $b;
        },
        'def' => function($name,$value) use(&$globalScope) {
            $globalScope[$name] = $value;
        },
        'do' => new SpecialForm('do',function() use ($localScope){
                $args = func_get_args();
                $retval = array_reduce($args,function($retval,$arg) use ($localScope) {
                        return lispEval($arg,$localScope);
                    });
                return $retval;
            }),
        'if' => new SpecialForm('if',function($condition,$success,$failure) use ($localScope){
                $passed = lispEval($condition,$localScope);
                if ($passed && (!is_null($success))) {
                    return lispEval($success,$localScope);
                } else if((!$passed) && (!is_null($failure))) {
                    return lispEval($failure,$localScope);
                }
            }),
        'fn' => new SpecialForm('fn',function($defArgs,$code) use ($localScope){
                $func = function() use($defArgs,$code,$localScope){
                    $args = [];
                    $_args = func_get_args();
                    foreach($defArgs as $index => $defArg) {
                        $args[$defArg] = lispEval($_args[$index],$localScope);
                    }
                    var_dump($args);
                    return lispEval($code,array_merge($args,$localScope));
                };
                return $func;
            }),
        'defn' => new SpecialForm('defn',function($name,$defArgs,$code) use($localScope){
                $code = ['def',$name,['fn',$defArgs,$code]];
                return lispEval($code,$localScope);
            }),
    ];

    $expression = array_map(function($symbol) use($scope){
            if ((!is_array($symbol)) && (array_key_exists($symbol,$scope))) {
                return $scope[$symbol];
            } else {
                return $symbol;
            }
        },$rawExpression);

    if ((is_array($expression)) && (count($expression) > 0))  {
        if (is_array($expression[0])) {
            $fn = lispEval($expression[0]);
        } elseif (array_key_exists($expression[0],$natives)) {
            $fn = $natives[$expression[0]];
        } else {
            $fn = $expression[0];
        }
        $args = array_map(function($arg) use($fn,$localScope){
                if (is_array($arg) && (!is_object($fn))) {
                    return lispEval($arg,$localScope);
                } else {
                    return $arg;
                }
            },array_slice($expression,1));
        return call_user_func_array($fn,$args);
    } else {
        return $expression;
    }        
}


$code = ["defn", "fib", ["n"],
         ["if", [">", "n", 1],
          ["+",
           ["fib", ["-", "n", 1]],
           ["fib", ["-", "n", 2]]],
          1]];

lispEval($code);

$code = ["sprintf", ["fib", 8]];

var_dump(lispEval($code));