如何在php中将图像上传到服务器从字符串更改为多部分？

I'm using android app to upload images to server, i want to change my php file from uploading images as strings to multipart.

I want to change because i have some problem that i can't upload more than 700KB of images on the server through my android application and i tried everything but still the same problem, someone adviced me to change to multipart because it better

So, could you help me to change this file to fit multipart, please?

<?php 
include "connection.php.php";

$sucess = 105;
$fail = 107;

$base = $_POST['sales_images'];
$filename = $_POST['sales_images_names'];


$bedroom = $_POST['bedroom'];
$bathroom = $_POST['bathroom'];
$size = $_POST['size'];

$ref = $_POST['ref'];

$hotDeal = $_POST['hotDeal'];

$sql = "INSERT INTO `sale`(`ref`, `sqm`, `avaliable`, `bedroom`, `bathroom`, `hot_deal`) VALUES ('$ref','$size',1,'$bedroom','$bathroom',$hotDeal)";
$result = mysqli_query($con, $sql);

if ($result === TRUE) {
    $last_id = $con->insert_id;

    $length = count($base);

    for($i = 0; $i < $length; $i++) {
        $imageNow = time();
        $new_imageFilename = $imageNow . "_" . $filename[$i];

        $sql6 = "INSERT INTO `sale_image`(`sale_id`, `img_url`) VALUES ('$last_id','$new_imageFilename')";
        $result6 = mysqli_query($con, $sql6);

        $binary=base64_decode($base[$i]);
        $file = fopen('files/sales/' . $new_imageFilename, 'wb');
        fwrite($file, $binary);
        fclose($file);      
    }

    echo "New record created successfully. Last inserted ID is: " . $last_id;
} else {
    echo "Error: " . $sql . "<br>" . $con->error;
}

$jsonResult = '{"state":';
if($result){
    $jsonResult.= $sucess;
}else {
    $jsonResult.= $fail;
}
$jsonResult .="}";
print_r($jsonResult);
?>