这是怎么回事PHP提交后无法保存到.txt文件中

mail.php

 <?php
$str=$_GET[[content];
$myfile = fopen("mail.txt", "w") or die("Unable to open file!");
fwrite($myfile, content);
fclose($myfile);
?>

提交邮箱可第一时间获取资讯

                      <form action="mail.php" method="post">
                        <input type="email" name="email" id="email" required="required" placeholder="Email" />
                        <label>
                        <input type="submit" value="提交" />
                        <div class="clear"></div>
                          </label>
                      </form>
                    </div>