PHP是一種廣泛使用的開源編程語言，不僅僅可以用于編寫動態網站，也可以用于服務器端腳本編寫。PHP在網絡開發領域非常流行，其中之一的功能就是文件上傳。

在PHP中，我們可以使用$_FILES來處理文件上傳。舉個例子：

<form action="upload.php" method="POST" enctype="multipart/form-data">
<input type="file" name="fileToUpload">
<input type="submit" value="上傳">
</form>

上述代碼片段是一個簡單的文件上傳表單，它將文件上傳到一個名為upload.php的服務器端腳本。以下是upload.php的代碼示例：

$target_dir = "uploads/";
$target_file = $target_dir . basename($_FILES["fileToUpload"]["name"]);
$uploadOk = 1;
$imageFileType = strtolower(pathinfo($target_file,PATHINFO_EXTENSION));
if(isset($_POST["submit"])) {
$check = getimagesize($_FILES["fileToUpload"]["tmp_name"]);
if($check !== false) {
echo "File is an image - " . $check["mime"] . ".";
$uploadOk = 1;
} else {
echo "File is not an image.";
$uploadOk = 0;
}
if (file_exists($target_file)) {
echo "Sorry, file already exists.";
$uploadOk = 0;
}
if ($_FILES["fileToUpload"]["size"] >500000) {
echo "Sorry, your file is too large.";
$uploadOk = 0;
}
if($imageFileType != "jpg" && $imageFileType != "png" && $imageFileType != "jpeg"
&& $imageFileType != "gif" ) {
echo "Sorry, only JPG, JPEG, PNG & GIF files are allowed.";
$uploadOk = 0;
}
if ($uploadOk == 0) {
echo "Sorry, your file was not uploaded.";
} else {
if (move_uploaded_file($_FILES["fileToUpload"]["tmp_name"], $target_file)) {
echo "The file ". basename( $_FILES["fileToUpload"]["name"]). " has been uploaded.";
} else {
echo "Sorry, there was an error uploading your file.";
}
}
}

上面的PHP腳本首先檢查上傳的文件是否是正確的類型和大小，并已經被正確地移動到名為"uploads/"的目錄。

總體而言，PHP文件上傳是一個非常方便的功能。只需幾行代碼就可以使用戶輕松地上傳文件到服務器。