function index(){

// 加密的参数
$array['APPID']     = '支付宝appid';
$array['RSA_PRIVATE'] = '支付宝key';
$array['orderNum']      = '订单编号';
$array['money']     = '订单交易金额';

$keyStr = '与客户端约定好的加密串';                        
$aes = new CryptAES();
$aes->set_key($keyStr);
$aes->require_pkcs5();
$encText = $aes->encrypt(json_encode($array));    
dump($encText);


echo json_encode(['result'=>'0','message'=>'即将跳转至支付页面！','content'=>$encText]);die;

}

源码

<?php
// +----------------------------------------------------------------------
// | ThinkPHP [ WE CAN DO IT JUST THINK IT ]
// +----------------------------------------------------------------------
// | Copyright (c) 2011 http://thinkphp.cn All rights reserved.
// +----------------------------------------------------------------------
// | Licensed ( http://www.apache.org/licenses/LICENSE-2.0 )
// +----------------------------------------------------------------------
// | Author: luofei614 <weibo.com/luofei614>　
// +----------------------------------------------------------------------
namespace Think;
/**
 * php aes加密类
 * 参数bit：格式，支持256、192、128，默认为128字节的
 * 参数mode：加密/解密方式，支持cfb、cbc、nofb、ofb、stream、ecb，默认为ecb
 * 参数key：密钥，默认为abcdefghijuklmno
 */
class CryptAES
{
    protected $cipher = MCRYPT_RIJNDAEL_128;
    protected $mode = MCRYPT_MODE_ECB;
    protected $pad_method = NULL;
    protected $secret_key = '';
    protected $iv = '';

    public function set_cipher($cipher)
    {
        $this->cipher = $cipher;
    }

    public function set_mode($mode)
    {
        $this->mode = $mode;
    }

    public function set_iv($iv)
    {
        $this->iv = $iv;
    }

    public function set_key($key)
    {
        $this->secret_key = $key;
    }

    public function require_pkcs5()
    {
        $this->pad_method = 'pkcs5';
    }

    protected function pad_or_unpad($str, $ext)
    {
        if ( is_null($this->pad_method) )
        {
            return $str;
        }
        else
        {
            $func_name = __CLASS__ . '::' . $this->pad_method . '_' . $ext . 'pad';
            if ( is_callable($func_name) )
            {
                $size = mcrypt_get_block_size($this->cipher, $this->mode);
                return call_user_func($func_name, $str, $size);
            }
        }
        return $str;
    }

    protected function pad($str)
    {
        return $this->pad_or_unpad($str, '');
    }

    protected function unpad($str)
    {
        return $this->pad_or_unpad($str, 'un');
    }

    public function encrypt($str)
    {
        $str = $this->pad($str);
        $td = mcrypt_module_open($this->cipher, '', $this->mode, '');

        if ( empty($this->iv) )
        {
            $iv = @mcrypt_create_iv(mcrypt_enc_get_iv_size($td), MCRYPT_RAND);
        }
        else
        {
            $iv = $this->iv;
        }

        mcrypt_generic_init($td, $this->secret_key, $iv);
        $cyper_text = mcrypt_generic($td, $str);
        $rt=base64_encode($cyper_text);
        //$rt = bin2hex($cyper_text);
        mcrypt_generic_deinit($td);
        mcrypt_module_close($td);

        return $rt;
    }

    public function decrypt($str){
        $td = mcrypt_module_open($this->cipher, '', $this->mode, '');

        if ( empty($this->iv) )
        {
            $iv = @mcrypt_create_iv(mcrypt_enc_get_iv_size($td), MCRYPT_RAND);
        }
        else
        {
            $iv = $this->iv;
        }

        mcrypt_generic_init($td, $this->secret_key, $iv);
        //$decrypted_text = mdecrypt_generic($td, self::hex2bin($str));
        $decrypted_text = mdecrypt_generic($td, base64_decode($str));
        $rt = $decrypted_text;
        mcrypt_generic_deinit($td);
        mcrypt_module_close($td);

        return $this->unpad($rt);
    }

    public static function hex2bin($hexdata) {
        $bindata = '';
        $length = strlen($hexdata);
        for ($i=0; $i< $length; $i += 2)
        {
            $bindata .= chr(hexdec(substr($hexdata, $i, 2)));
        }
        return $bindata;
    }

    public static function pkcs5_pad($text, $blocksize)
    {
        $pad = $blocksize - (strlen($text) % $blocksize);
        return $text . str_repeat(chr($pad), $pad);
    }

    public static function pkcs5_unpad($text)
    {
        $pad = ord($text{strlen($text) - 1});
        if ($pad > strlen($text)) return false;
        if (strspn($text, chr($pad), strlen($text) - $pad) != $pad) return false;
        return substr($text, 0, -1 * $pad);
    }
}