如何在PHP中处理文件上传？

在PHP中可以使用$_FILES超全局变量处理文件上传。以下是一个文件上传的示例代码：

<?php
if(isset($_FILES["fileToUpload"])) {
    $targetDir = "uploads/";
    $targetFile = $targetDir . basename($_FILES["fileToUpload"]["name"]);
    $uploadOk = 1;
    $imageFileType = strtolower(pathinfo($targetFile,PATHINFO_EXTENSION));

    // Check if image file is a actual image or fake image
    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;
        }
    }

    // Check if file already exists
    if (file_exists($targetFile)) {
        echo "Sorry, file already exists.";
        $uploadOk = 0;
    }

    // Check file size
    if ($_FILES["fileToUpload"]["size"] > 500000) {
        echo "Sorry, your file is too large.";
        $uploadOk = 0;
    }

    // Allow certain file formats
    if($imageFileType != "jpg" && $imageFileType != "png" && $imageFileType != "jpeg"
    && $imageFileType != "gif" ) {
        echo "Sorry, only JPG, JPEG, PNG & GIF files are allowed.";
        $uploadOk = 0;
    }

    // Check if $uploadOk is set to 0 by an error
    if ($uploadOk == 0) {
        echo "Sorry, your file was not uploaded.";
    // if everything is ok, try to upload file
    } else {
        if (move_uploaded_file($_FILES["fileToUpload"]["tmp_name"], $targetFile)) {
            echo "The file ". basename( $_FILES["fileToUpload"]["name"]). " has been uploaded.";
        } else {
            echo "Sorry, there was an error uploading your file.";
        }
    }
}
?>
<!-- 前端的HTML表单 -->
<form action="fileUpload.php" method="post" enctype="multipart/form-data">
    Select image to upload:
    <input type="file" name="fileToUpload" id="fileToUpload">
    <input type="submit" value="Upload Image" name="submit">
</form>

在此示例代码中，PHP脚本首先检查所上传的文件是否符合指定的规则（比如文件大小、扩展名等）。如果文件符合规定，则将文件移动到uploads目录下。如果文件上传成功，则返回成功信息“ The file has been uploaded. ”。否则，返回错误信息“ Sorry, there was an error uploading your file.”。