Generate random strings that can be expanded by yourself

Generate a random string, which can be expanded by yourself
$type can be: upper (only generates uppercase letters), lower (only generates lowercase letters), number (only generates numbers)
$len is the length, defining the length of the string

/***************************
* Generate random strings and can be expanded by yourself
* $type can be: upper (only uppercase letters are generated), lower (only lowercase letters are generated), number (only numbers are generated)
* $len is the length, defining the string length
************************ ****/
function _random($type,$len)
{
 $new = '';
 $string = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ'; //Random number pool
 if($type= ='upper')
 {
 for($i = 0;$i<$len;$i++)
 {
 $new .= $string[mt_rand(36,61)];
 }
 return $new;
 }
 if($type == 'lower')
 {
 for($i = 0;$i<$len;$i++)
 {
 $new .= $string[mt_rand(10,35)];
 }
 return $new;
 }
 if($type == 'number')
 {
 for($i = 0;$i<$len;$i++)
 {
 $new .= $string[mt_rand( 0,9)];
 }
 return $new;
 }
}