您忘记了enctype=“multipart/form-data”
尝试此版本

<form action="upload.php" method="post" enctype="multipart/form-data">
  <p><input type="file" name="file">
  <p><button type="submit">Submit</button>
</form>
</body>
</html>

您没有提到上传的文件类型。下面是一个上传图像的示例。允许选择多个图像。

<form action="upload.php" method="post" enctype="multipart/form-data">
Get a Photo from your device <br>
<input type="file" name="image1[]" multiple accept="image/png, image/jpeg, image/gif, image/webp" /><br>
<button type="submit">Upload Photo</button>
</form>

和上传PHP：

foreach ($_FILES["image1"]["error"] as $key => $error) {
    if ($error == 0 ) {
        $tmp_name = $_FILES["image1"]["tmp_name"][$key];
        $name = basename($_FILES["image1"]["name"][$key]);
        $result = move_uploaded_file($tmp_name, "$uploads_dir/$name");
        if($result == false){echo "not moved\n";}
    }
    if($error > 0){echo "error $error\n";}
}

要确定上传的图像类型，请执行以下操作：

switch(strtolower($_FILES['image1']['type'])){
  case 'image/jpeg':
    $image = @imagecreatefromjpeg($_FILES['image1']['tmp_name']);
    if ($image !== false){$save = true;break;}
  case 'image/png':
    $image = @imagecreatefrompng($_FILES['image1']['tmp_name']);
    if ($image !== false){$save = true;break;}
  case 'image/gif':
    $image = @imagecreatefromgif($_FILES['image1']['tmp_name']);
    if ($image !== false){$save = true;break;}
  case 'image/webp':
    $image = @imagecreatefromwebp($_FILES['image1']['tmp_name']);
    if ($image !== false){$save = true;break;}
  default:
    $img = @getimagesize($_FILES['image1']['tmp_name']);
    switch(strtolower($img['mime'])){
    case 'image/jpeg':
      $image = @imagecreatefromjpeg($_FILES['image1']['tmp_name']);
      if ($image !== false){$save = true;break;}
    case 'image/png':
      $image = @imagecreatefrompng($_FILES['image1']['tmp_name']);
      if ($image !== false){$save = true;break;}
    case 'image/gif':
      $image = @imagecreatefromgif($_FILES['image1']['tmp_name']);
      if ($image !== false){$save = true;break;}
    default:
      $filename = $_FILES['image1']['name'];
      $ext = substr($filename,-3);
      switch(strtolower($ext)){
      case 'jpg':
        $image = @imagecreatefromjpeg($_FILES['image1']['tmp_name']);
        if ($image !== false){$save = true;break;}
      case 'ebp':
        $image = @imagecreatefromwebp($_FILES['image1']['tmp_name']);
        if ($image !== false){$save = true;break;}
      case 'gif':
        $image = @imagecreatefromgif($_FILES['image1']['tmp_name']);
        if ($image !== false){$save = true;break;}
      case 'png':
        $image = @imagecreatefrompng($_FILES['image1']['tmp_name']);
        if ($image !== false){$save = true;break;}
      default:
        $image = @imagecreatefromjpeg($_FILES['image1']['tmp_name']);
        if ($image !== false){$save = true;break;}
        $image = @imagecreatefrompng($_FILES['image1']['tmp_name']);
        if ($image !== false){$save = true;break;}
        $image = @imagecreatefromgif($_FILES['image1']['tmp_name']);
        if ($image !== false){$save = true;break;}
      }
    }
  }

假设你已经可以上传文件了，比如使用如下形式：

<form action="upload.php" method="post" enctype="multipart/form-data">
  <p><input type="file" name="file">
  <p><button type="submit">Submit</button>
</form>
</body>
</html>

然后，如果你想在接收端“upload.php”显示图像，你可以用途：
如果图像为JPG/JPEG类型：

<?php
$string1= base64_encode(file_get_contents($_FILES["file"]["tmp_name"]));

echo '<img src="data:image/jpeg;base64, ' .  $string1  . '"/>';

?>

如果图像为PNG类型：

<?php
$string1= base64_encode(file_get_contents($_FILES["file"]["tmp_name"]));

echo '<img src="data:image/png;base64, ' .  $string1  . '"/>';

?>

注意：实际上我们通常把上传的文件保存在服务器中（以备以后显示），或者把二进制数据存储到db表中的BLOB字段中，但上面的代码是为了演示如何做才能立即显示上传的图片。