How do I implement server-side textbox and checkbox validation using PHP?

I'm currently working on a program where I have to validate a form on the server-side using PHP. I've been able to write working validation code for and fields but I'm a bit stuck on some parts. I'm pretty new to coding and I'm not sure how to validate that a textbox contains a certain string using PHP. I've tried Googling but nothing seems to be working. Also, I need to verify a checkbox, where certain options from a dropdown only become active when a user checks the checkbox. I was sort of able to do this, but only after the user clicks the submit button were the changes applied. I need the changes to apply as soon as they click the checkbox.

I tried this for the textbox:

 $a = "June";
 $b = "July";
 if (empty($_POST[""])) {
    $textErr = "Text is required.";
} else if (!(preg_match("/{$a}/i", $text)) && (!preg_match("/{$b}/i", $text))){
    $textErr = "Text must contain either 'June' or 'July'.";
}
else {
    $text= test_input($_POST["text"]);
}

and also this:

$checkjune = stripos($_POST["text"], $a);
$checkjuly = stripos($_POST["text"], $b);

 (empty($_POST["text"])) {
    $textErr = "Text is required.";
} else if ($checkfl === false) {
    $textErr = "Text must contain either 'June' or 'July'.";
} else {
    $text = test_input($_POST["text"]);
}

This is kinda what I have currently for the checkbox:

(in HTML code)

 <div class="row">
                <input id="student" name="student" type="checkbox">I am a student</input>
            </div>
            <div class="row">
                <label for="subject">Favorite Subject: </label>
                <select id="subject" name="subject" >
                    <option value="0" enabled>Not Applicable (I am not a student) </option>
                    <option value="1" disabled>Math</option>
                    <option value="2" disabled>Science</option>
                    <option value="3" disabled>English</option>
                   
                    <?php
                        if(!empty($_POST['student'])) {
                    echo "<script>document.getElementById('subject').disabled = ''; </script>";
                             }
                     ?>
                </select>
            </div>