PHP Post superglobal Array 不包含表单中的所有键

我的php脚本中有这个html表单

<form action="" method="post">
<div style="width: 30em;">
    <label for="firstName">First name *</label>
    <input type="text" name="firstName" id="firstName" value=""/>
    <label for="lastName">Last name *</label>
    <input type="text" name="lastName" id="lastName" value=""/>
    <label for="password1">Choose a password *</label>
    <input type="password" name="password1" id="password1" value=""/>
    <label for="password2">Retype password *</label>
    <input type="password" name="password2" id="password2" value=""/>
    <label>Your gender: *</label>
    <label for="genderMale">Male</label>
    <input type="radio" name="gender1" id="genderMale" value="M" />
    <label for="genderFemale">Female</label>
    <input type="radio" name="gender" id="genderFemale" value="F" />
    <label for="favoriteWidget">What's your favorite widget? *</label>
    <select name="favoriteWidget" id="favoriteWidget" size="1">
        <option value="superWidget">The SuperWidget</option>
        <option value="megaWidget">The MegaWidget</option>
        <option value="wonderWidget">The WonderWidget
        </option>
    </select>
    <label for="newsletter">Do you want to receive our newsletter?</label>
    <input type="checkbox" name="newsletter" id="newsletter" value="yes"/>
    <label for="comments">Any comments?</label>
    <textarea name="comments" id="comments" rows="4" cols="50"></textarea>
    <div style="clear: both;">
        <input type="submit" name="submitButton" id="submitButton" value="Send Details"/>
        <input type="reset" name="resetButton" id="resetButton" value="Reset Form" style="margin-right: 20px;"/>
    </div>
</div>
</form>

问题是当我提交表单并运行此 php 代码时：

if ( isset($_POST["submitButton"]) ) {
     echo '<pre>' . print_r($_POST, true) . '</pre>';  
}

我得到以下结果：

Array
(
    [firstName] => 
    [lastName] => 
    [password1] => 
    [password2] => 
    [favoriteWidget] => superWidget
    [comments] => 
    [submitButton] => Send Details
)

最明显的是 $_POST 数组中的性别和新闻通讯键丢失了？这可能是什么原因呢？