Custom function to generate random password in php

/*
 * php automatically generates new password custom function (with example demonstration)
 Applicable environment: PHP5.2.x / mysql 5.0.x
 Code author: xujiajay
 Contact information: xujiaphp@gmail.com
* */
function genPassword($min = 5, $max = 8)
{
 $validchars="abcdefghijklmnopqrstuvwxyz123456789";
 $max_char=strlen($validchars)-1;
 $length=mt_rand ($min,$max);
 $password = "";
 for($i=0;$i<$length;$i++ )
 {
 $password.=$validchars[mt_rand(0,$max_char)] ;
 }
 return $password;
 }
 echo "New password:".genPassword()."";
 echo "New password:".genPassword(5,10)."" ;
?>