我怎么会偶尔有生成三位验证码的情况

for($i=0;$i<4;$i++){//随机4个字符

$fontsize = 6;

$fontcolor = imagecolorallocate($image,rand(0,120),rand(0,120),rand(0,120));

$data = '0123456789abcdefghijklmnopqrstuvwxyz';

$fontcontent = substr($data,rand(0,strlen($data)),1);

$captch_code.=$fontcontent;

$x = ($i*100/4)+(rand(5,10));

$y = rand(5,10);

imagestring($image,$fontsize,$x,$y,$fontcontent,$fontcolor);

}