$mesanterior = date("F", mktime(0, 0, 0, date("m")-1,date("d"),date("Y")));
en español:
Código PHP:
<?php
setlocale(LC_ALL,"es-ES");
$loc = setlocale(LC_TIME, NULL);
$mesanterior = strftime("%B", mktime(0, 0, 0, date("m")-1,date("d"),date("Y")));
echo $mesanterior;
?>