目标c -如何在PHP中解密post数据

我在Objective-c中加密了一些数据，代码如下:

NSDictionary *jsonDic = [NSDictionary dictionaryWithObjectsAndKeys:@"1", @"key1", @"2", @"key2", nil]; 
SBJsonWriter *jsonWriter = [[SBJsonWriter alloc] init];
NSData *data = [jsonWriter dataWithObject:jsonDic];
[jsonWriter release];
NSString *key = @"mykey";   
const int len = [data length]*2 + 2048;
char* buffer = (char*)malloc(len);
memset(buffer, 0, len);
const char *iv = "12345";
size_t dataOut = 0;
CCCryptorStatus cryptStatus = CCCrypt(kCCEncrypt, 
                                      kCCAlgorithmDES,
                                      kCCOptionPKCS7Padding,
                                      [key UTF8String], 
                                      kCCKeySizeDES,
                                      iv, 
                                      [data bytes],
                                      [data length],
                                      buffer, 
                                      len,
                                      &dataOut);
NSData* retData = nil;
if(cryptStatus == kCCSuccess) {
    retData = [NSData dataWithBytes:buffer length:dataOut];
}
free(buffer);
return destData;

然后调用以下代码post到php api

NSString *urlString = @"http://domain/api.php";
NSMutableURLRequest *urlPost = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:url] cachePolicy:NSURLRequestUseProtocolCachePolicy timeoutInterval:20];
[urlPost setHTTPMethod:@"POST"];
[urlPost setHTTPBody:data];
NSURLConnection *urlConnection = [NSURLConnection connectionWithRequest:urlPost delegate:self];

我使用下面的php代码读取和解密:

define("KEY","mykey");
define("IV","12345");
$post_body = file_get_contents('php://input');
$dec_str   = mcrypt_decrypt(MCRYPT_DES, KEY, $post_body, MCRYPT_MODE_BCB, IV);
error_log($dec_str);

但是我看不到正确的结果。经过一天一夜的寻找和尝试，仍然没有发现哪里出了问题。谢谢提前!