Php form repeat button and null input

hello,
i know the title looks little bit strange but i have an issue with my php form , after i submit with empty input box i don’t get the echo results as wanted in the code with unanswered mark , as well when put right answer it gives back blank page after submitting . one more thing how can i add the button of repeat the exercise after submitting so it gives the form again for do the exercise over again, and how as well the button of showing results without taking the exam , means press the show results button shows them directly and thank you all in advance

this is my code

<?php
$solution="do";
$solution3="do1";
if (isset($_POST['solution1'])){
  $solution1=$_POST['solution1'];
  if ($_POST['add']){
    if ($solution1 != $solution){
      echo '<span style="color: red;">'.$solution1.'</span> you like lemon?</br>';
    }
  } else if ($solution1 = null) { 
    echo '<span style="color: blue;">"unanswered"</span> you like lemon?</br>';
  } else { 
    echo '<span style="color: green;">'.$solution1.'</span> you like lemon?</br>';
  }
}
if (isset($_POST['solution2'])){
  $solution2=$_POST['solution2'];
  if ($_POST['add']){
    if ($solution2 != $solution3){
      echo '<span style="color: red;">'.$solution2.'</span> you like lemon?</br>' ;
    }
  } else if ($solution2 = null) { 
    echo '<span style="color: blue;">"unanswered"</span> you like lemon?</br>';
  } else { 
    echo '<span style="color: green;">'.$solution2.'</span> you like lemon?</br>';
  }
}
else {
?>
        <form   method="post" autocomplete="off" >
            <input type="text"  name="solution1" /> you like lemon?
            </br>
            <input type="text"  name="solution2" /> you like apple?
  
            <input  type="submit" name="add" value="Check" />
        </form>
<?php
}
?>

I added some indentation to your code, to make it easier to read and understand.

if ($solution1 = null) { 

this line (idem for $solution2) is always true, because you’re assigning a value (=) instead of confronting two values (==)

ok thank you, im still waiting for help guys

After correcting the error I pointed out, do you still have the same problems as before?