An example of how one might use https://github.com/nervgh/angular-file-upload to communicate with a PHP proxy and Amazon S3.

app.js

(function() {
    var module = angular.module('myApp', [
        'angularFileUpload'
    ]);

    module.controller('MyCtrl', ['$scope', 'FileUploader', function($scope, FileUploader) {
        'use strict';

        $scope.uploader = new FileUploader({
            url                    : 'http://mydomain.com/s3proxy.php',
            filters                : [
                {
                    name : 'imageFilter',
                    fn   : function(item /*{File|FileLikeObject}*/, options) {
                        var type = '|' + item.type.slice(item.type.lastIndexOf('/') + 1) + '|';
                        return '|jpg|jpeg|png|'.indexOf(type) !== -1;
                    }
                }
            ],
            onWhenAddingFileFailed : function(item /*{File|FileLikeObject}*/, filter, options) {},
            onAfterAddingFile      : function(fileItem) {
                fileItem.upload();
            },
            onAfterAddingAll       : function(addedFileItems) {},
            onBeforeUploadItem     : function(item) {},
            onProgressItem         : function(fileItem, progress) {},
            onProgressAll          : function(progress) {},
            onSuccessItem          : function(fileItem, response, status, headers) {
                fileItem.s3url = response.data.url;
            },
            onErrorItem            : function(fileItem, response, status, headers) {},
            onCancelItem           : function(fileItem, response, status, headers) {},
            onCompleteItem         : function(fileItem, response, status, headers) {},
            onCompleteAll          : function() {}
        });
    
    }]);
    
})();

composer.json

{
    "require": {
        "aws/aws-sdk-php": "~2.7.0"
    }
}

s3proxy.php

<?php

require_once('vendor/autoload.php');

function handleRequest() {
    header( 'Expires: ' . date( 'Y-m-d H:i:s' ) );
    header( 'Pragma: no-cache' );
    header( 'Cache-control: value="no-cache, no-store, must-revalidate' );
    header( 'x-frame-options: sameorigin' );
    header( 'Content-type: application/json; charset=utf-8' );
    
    $count = count( $_FILES );

    if ( $count === 1 ) {
        $file = $_FILES['file'];
        $data = uploadFileToS3( $file );
        success( $data );
    }
    else {
        $message = sprintf( 'One file required per request, this request contained %s files.', $count );
        badRequest( $message );
    }
}

function uploadFileToS3() {
    $aws_access_key_id = '';
    $aws_secret_access_key = '';
    $bucket = 'my-bucket';
    $prefix = 'my-app/';
    $acl    = 'public-read';
    
    $errors = validate( $file );
    if ( count( $errors ) > 0 ) badRequest( null, $errors );

    $credentials = array(
        'key'    => $aws_access_key_id,
        'secret' => $aws_secret_access_key
    );
    $client = S3Client::factory( $credentials );
    
    $object = array(
        'Bucket'     => $bucket,
        'Key'        => $prefix . $file['name'],
        'SourceFile' => $file['tmp_name'],
        'ACL'        => $acl
    );
    $result = $client->putObject( $object );
    
    return $result->asArray();
}

function validate( array $file ) {
    $max_file_size = 2097152;
    $accepted_file_types = array( 'jpg', 'jpeg', 'png' );
    
    $file_extension = strtolower( substr( strrchr( ( $file['name'] ), '.' ), 1 ) );
    $errors = array();
    
    if ( $file['name'] == '' )
        $errors['name'] = 'Missing file name';
    if ( $file['error'] )
        $errors['upload'] = sprintf( 'Error uploading file: %s', $file['error'] );
    if ( $file['size'] > $max_file_size )
        $errors['size'] = sprintf( 'File size exceeds maximum of %s bytes', $max_file_size );
    if ( ! in_array( $file_extension, $accepted_file_types ) )
        $errors['type'] = sprintf( 'Only the following file types are accepted: %s', $accepted_file_types );

    return $errors;
}

function badRequest( $message = null, array $errors = null ) {
    header( 'HTTP/1.1 400 Bad Request' );
    $response = array('success' => false);
    
    if ( isset( $message ) )
        $response['message'] = $message;
    if ( isset( $errors ) )
        $response['errors'] = $errors;
    
    exit( json_encode( $response ) );
}

function success( array $data ) {
    $response = array('success' => true);
    
    if ( isset( $data ) )  {
        header( 'HTTP/1.1 200 OK' );
        $response['data'] = $data;
    }
    else {
        header( 'HTTP/1.1 204 No Content' );
    }
    
    exit(json_encode($response));
}