Confusion over the value of a variable after running a PHP program

I'm studying for my finals and I came across this question:

consider this following PHP code, write the output after executing it

<?php
  $a=3;
  $b=$a++;
  IF($a>$b)
  {
    echo "a>$b";
  }
  else if ($a == $b)
  {
    echo "a=$b";
  }
  else
  {
    echo "a < $b";
  }
  ?>
 

When I output it in my text editor I get a < 3, but I don't understand why though?

I thought a is assigned to 3 and also b is assigned to a++ 3 and 3==3 so should a==3 be printed out?

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