why it consists errors even though it's correct

Question

<?php

$num1=$_POST['num1'];

$num2=$_POST['num2'];

$func=$_POST['func'];

$submit=$_POST['submit'];

if(submit == false ){

include ('cool.php');

}

else if ($func == add){

include ('cool.php');

$num3=$num1+$num2;

echo"<b><b>";

echo"<b>solution</b>:";

echo"<b>";

echo"$num1+$num2=$num3";

}else if ($func == sub)

{

include ('cool.php');

$num3=$num1-$num2;

echo"<b><b>";

echo"<b>solution</b>:";

echo"<b>";

echo"$num1-$num2=$num3";

}else if ($func == mul)

{

include ('cool.php');

$num3=$num1*$num2;

echo"<b><b>";

echo"<b>solution</b>:";

echo"<b>";

echo"$num1 x/*$num2=$num3";

}else if ($func == div)

{

include ('cool.php');

$num3=$num1/$num2;

echo"<b><b>";

echo"<b>solution</b>:";

echo"<b>";

echo"$num1/$num2=$num3";

};

?>

the output has errors even though I'm sure it's correct plz help me out 

the errors 

Notice: Use of undefined constant submit - assumed 'submit' in C:\wamp\www\the test\verb.php on line 7
Call Stack
#	Time	Memory	Function	Location
1	0.0380	147384	{main}( )	..\verb.php:0

( ! ) Notice: Use of undefined constant add - assumed 'add' in C:\wamp\www\the test\verb.php on line 10
Call Stack
#	Time	Memory	Function	Location
1	0.0380	147384	{main}( )	..\verb.php:0