Imaginary numbers<br><b>0, 0i</b>
<form method="post">
<input name="a" size="5" placeholder="Real" value="<? echo $_POST['a']; ?>"><input name="b" size="5" placeholder="Imagine" value="<? echo $_POST['b']; ?>"/><br>
<input name="c" size="5" placeholder="Real" value="<? echo $_POST['c']; ?>"><input name="d" size="5" placeholder="Imagine" value="<? echo $_POST['d']; ?>"/><br>
<style>button,input{background:#070;color:#0ff;placeholder-color:#fff;padding:0.2em;margin:0.2em}

::placeholder { /* Chrome, Firefox, Opera, Safari 10.1+ */
  color: #ff8; opacity: 1;} /* Firefox */
::-ms-input-placeholder { /* Microsoft Edge */ color: #ff8;}
*{background:#000;color:#0f0;}b{color:#0cf}
</style>
<br><br>
<button name="op" value="1" type="submit">Add</button>
<button name="op" value="2" type="submit">Subtract</button><br>
<button name="op" value="3" type="submit">Multiply</button>
<button name="op" value="4" type="submit">Divide</button><br>
<button name="op" value="5" type="submit">Power</button>
<button name="op" value="7" type="submit">Log</button><br>
<button name="op" value="6" type="submit">Mod</button>
<button name="op" value="8" type="submit">Polar and Rectangular</button>

</form>
<p>The top values can be converted to rectangular, and the bottom values to polar, by using the polar to rectangular button (new).</p>
<p>I didn't want to implement trigonometry functions as they will need to either return 2 separate values, or add/combine the values.</p>