execute方法通过eval执行JS代码。eval好像Mongo 3里是deprecated。eval也用个global write lock，最好不用。

你可以用command方法，PHP.net上有个mapReduce例子（http://php.net/manual/en/mong...），下面给你参考：

// construct map and reduce functions
$map = new MongoCode("function() { emit(this.user_id,1); }");
$reduce = new MongoCode("function(k, vals) { ".
    "var sum = 0;".
    "for (var i in vals) {".
        "sum += vals[i];". 
    "}".
    "return sum; }");

$sales = $db->command(array(
    "mapreduce" => "events", 
    "map" => $map,
    "reduce" => $reduce,
    "query" => array("type" => "sale"),
    "out" => array("merge" => "eventCounts")));

但是我建议这样简单的mapreduce可以通过aggregation framework写出来。