jgn ganti logika donk

 <select name="bln" required>

<?php
$blnLahir = 'Maret'; // misalkan bulan nya 8=
$bln=array(1=>"Januari","Februari","Maret","April","Mei","Juni","July","Agustus","September","Oktober","November","Desember");
for($bulan=1; $bulan<=12; $bulan++){
$tmp= "<option %s value='%s'>%02s</option>";
//$selected= (int)$blnLahir[$bulan]==$bln[$bulan]?'selected':''; // Bingung di sini
$selected=  strtolower($blnLahir) == strtolower($bln[$bulan])?'selected':'';
$title=$bln[$bulan];
$val=$title;
 printf($tmp, $selected, $val, $title);
}

// $bln=array(1=>"Januari","Februari","Maret","April","Mei","Juni","July","Agustus","September","Oktober","November","Desember");
// for($bulan=1; $bulan<=12; $bulan++){
//  echo "<option value='$bln[$bulan]'>$bln[$bulan]</option>";
?>
</select>