I'm trying:

$m = 'December';
$from = date('jS M Y', strtotime("first day of previous $m"));

But it's not returning a date. In use, the month will be selected by a user from a dropdown box. However, I have tried typing 'December' in place of the variable directly and it still doesn't work.

I'm expecting it to return "1st Dec 2012".

Given your code that $m is the current month in text format, you can use something like:

$m = 'June';
$m = date('m', strtotime($m));
$c = (mktime(0,0,0,$m, 1) < time()) ? mktime(0,0,0,$m,1) : mktime(0,0,0,$m,1, date("Y") -1);
$from = date('jS M Y', $c);

What this does is it converts the month to a numerical value and stores in place of string value. Then it assigns $c to either the month of the current year, or the month of the previous year, depending on if the month of the current year is less than the current time stamp. So if the current timestamp is June 1st at 1am, then the current year will still be selected.

DEMO

Try this

$m = '12'; // set here numeric form of month
echo $from = date('jS M Y', strtotime("01-".$m."-".(date("Y") -1)));

OR

$m = 'December';
echo $from = date('jS M Y', strtotime("01-".$m."-".(date("Y") -1)));

Output

1st Dec 2012 

Codepad

EDIT

Try this, this is tested for january as well as december

$m = '01';
$new_m = date('m'); // get current month
if($m > $new_m) {
   $year = date("Y") -1;
} else {
   $year = date("Y");
}
echo $from = date('jS M Y', strtotime("01-".$m."-".$year));

Output

1st Jan 2013 

Codepad