ajax代码不调用php文件，html表单

function show(value) {
    var datastring = 'ptcid='+ value

    $.ajax({

        type: "POST",
        url: "filldropdown.php",
        data: datastring,
        success: function(responseText){$("#ptscid").html(responseText);}, 
        error : function  { (alert("error"); )}
    });
}

this is the code of select option which is dependent on another select option

my problem is that filldropdown file is not called. even i checked apache access log, it doesn"t show any access of filldropdown.php file. how to make ajax access it

Your

var datastring = 'ptcid='+ value

should be

var datastring = 'ptcid='+ value;

you must have semicolon ; at the end of JS statement.

EDIT:

Also, you must declare function properly in AJAX error

error: function(){alert("error");}

and you don't need to close alert in () only argument to alert() function.