用于随机显示图片的PHP代码，可自行改编成函数。

<?php
/**
 * Filename : randomimg.php
 * Usage:
 *   <img src="randomimg.php" />
 *   <img src="randomimg.php?folder=pic" />
 */

define('WORK_ROOT', dirname(__FILE__));

if(@$_GET['folder']){
    $folder=$_GET['folder'];
} else {
    $folder='pic';
}

$path = WORK_ROOT . '/' . $folder;

$files=array();

if ($dir = opendir($path)){

    while (($file = readdir($dir)) !== false){
        
        if(preg_match('/.gif$|.jpg$|.png$/',$file)){
            //echo "filename: " . $file . ", 
";
            $files[count($files)] = $file;
        }
    
    }
  closedir($dir);
}
 
//echo var_dump($files);

$random=rand(0,count($files)-1);
//echo $files[$random];

if(substr($files[$random],-3)=='gif'){
    header("Content-type: image/gif");
}else if(substr($files[$random],-3)=='jpg'){
    header("Content-type: image/jpeg");
}else if(substr($files[$random],-3)=='png'){
    header("Content-type: image/png");
}

readfile("$path/$files[$random]");

Try It Yourself： https://liaosnet.com/tiy/tiy_randompic.php
注：由于每次均需要对目录进行操作，效率并不高，建议仅用于了解随机方法。