$time_array = getdate();

$year = $time_array['year'];
$month = $time_array['mon'];  //I do believe this starts at 0, check

if($month == 0){
    $year--;
    $month = 11;
}
else{
    $month--;
}

$formatted = sprintf("%04d%02d", $year, $month); //please check
print $formatted; 

<?php
function pGetLM_yyyymm()
{
  $ta = getdate();
    
  $year = $ta['year'];
  $month = $ta['mon'];
    
  --$month; // go back 1 month
    
  if( $month === 0 ): // if this month is Jan
    --$year; // go back a year
    $month = 12; // last month is Dec
  endif;
    
  // return in format 'YYYYMM'
  return sprintf( $year.'%02d', $month); 
}

?>