Check if $_POST exists

Simple. You've two choices:

1. Check if there's ANY post data at all

//Note: This resolves as true even if all $_POST values are empty strings
if (!empty($_POST))
{
    // handle post data
    $fromPerson = '+from%3A'.$_POST['fromPerson'];
    echo $fromPerson;
}

(OR)

2. Only check if a PARTICULAR Key is available in post data

if (isset($_POST['fromPerson']) )
{
    $fromPerson = '+from%3A'.$_POST['fromPerson'];
    echo $fromPerson;
}

Everyone is saying to use isset() - which will probably work for you.

However, it's important that you understand the difference between

$_POST['x'] = NULL; and $_POST['x'] = '';

isset($_POST['x']) will return false on the first example, but will return true on the second one even though if you tried to print either one, both would return a blank value.

If your $_POST is coming from a user-inputted field/form and is left blank, I BELIEVE (I am not 100% certain on this though) that the value will be "" but NOT NULL.

Even if that assumption is incorrect (someone please correct me if I'm wrong!) the above is still good to know for future use.

if( isset($_POST['fromPerson']) )
{
     $fromPerson = '+from%3A'.$_POST['fromPerson'];
     echo $fromPerson;
}

Surprised it has not been mentioned

if($_SERVER['REQUEST_METHOD'] == 'POST' && isset($_POST['fromPerson'])){