我的AngularJS项目中未检测到PHP文件

I have an AngularJS app which has a mailer script written in PHP. I'm trying to refer to the PHP file via the angular $http service but I keep getting a 404 error when I try to use it via my contact form, in the corresponding controller, like so:

angular.module('myModule')
.controller('contactUsController', ['$scope', '$http', function ($scope, $http) {
    $scope.formData = {};
    $scope.submitted = false;
    $scope.submit = function(contactform) {
        console.log('Form data', $scope.formData);
        $scope.submitted = false;
        $scope.submitButtonDisabled = true;
        if (contactform.$valid) {
            $http({
                method  : 'POST',
                url     :  "app/process.php",
                data    : $.param($scope.formData),  
                headers : { 'Content-Type': 'application/x-www-form-urlencoded' } 
            }).success(function(data){
                console.log(data);
                if (!data.success) {
                    // if not successful, bind errors to error variables
                    $scope.errorName = data.errors.name;
                    $scope.errorEmail = data.errors.email;
                    $scope.errorTextarea = data.errors.message;
                    $scope.submissionMessage = "Sorry. Error sending message. Please try again.";
                    $scope.submission = true; //shows the error message
                } else {
                    // if successful, bind success message to message
                    $scope.formData = {}; // form fields are emptied with this line
                    $scope.submissionMessage = "Thank you! Your message has been sent successfully.";
                    $scope.submitted = true; //shows the success message
                }
            });
        } else {
        }
    }
}]);

So each time I invoke the submit() function by pressing the send button, the browser complains like so:

I've been searching around for an answer, but I haven't found one that could help me out.

I am using npm start to run my app. My project structure is as shown in the image below:

Any idea what could be going wrong? Any help is appreciated.