php生成图片验证码

<?php 
/**
 * 生成验证码
 * @param int $type
 * @param int $length
 * @return string
 */
function buildRandomString($type=1,$length=4){
    if ($type == 1) {
        //join()函数是implode()函数的别名，用来将数组变成字符串，第一个参数为数组元素之间放置的内容
        $chars = join ( "", range ( 0, 9 ) );
    } elseif ($type == 2) {                     
        //range()函数创建一个包含指定范围的元素的数组
        //array_merge()函数把一个或多个数组合并为一个数组
        $chars = join ( "", array_merge ( range ( "a", "z" ), range ( "A", "Z" ) ) );
    } elseif ($type == 3) {
        $chars = join ( "", array_merge ( range ( "a", "z" ), range ( "A", "Z" ), range ( 0, 9 ) ) );
    }
    if ($length > strlen ( $chars )) {
        exit ( "字符串长度不够" );
    }
    $chars = str_shuffle ( $chars );//str_shuffle()函数随机打乱字符串中的所有字符
    return substr ( $chars, 0, $length );//substr()函数返回指定部分的字符串
}