為了保證上傳圖片或檔案的完整性，通常客戶端和服務端會對圖片進行md5校驗：

iOS 端：//MARK:大檔案的MD5加密
-(NSString*)fileMD5:(NSString*)path
{
    NSFileHandle *handle = [NSFileHandle fileHandleForReadingAtPath:path];
    if( handle== nil ) return @"ERROR GETTING FILE MD5"; // file didnt exist
    CC_MD5_CTX md5;
    CC_MD5_Init(&md5);
    BOOL done = NO;
    while(!done)
    {
        NSData* fileData = [handle readDataOfLength:256];
        //        CHUNK_SIZE
        CC_MD5_Update(&md5, [fileData bytes], [fileData length]);
        if( [fileData length] == 0 ) done = YES;
    }
    unsigned char digest[CC_MD5_DIGEST_LENGTH];
    CC_MD5_Final(digest, &md5);
    NSString* s = [NSString stringWithFormat: @"%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x%02x",
                   digest[0], digest[1],
                   digest[2], digest[3],
                   digest[4], digest[5],
                   digest[6], digest[7],
                   digest[8], digest[9],
                   digest[10], digest[11],
                   digest[12], digest[13],
                   digest[14], digest[15]];
    return s;
}

   NSString * md5Path = @"檔案的本地地址";
    NSString * resultMD5 = [self fileMD5:md5Path];
    //對比MD5
    NSLog(@"md5:%@",resultMD5); //md5:b281bf08824cfc0668b42cf96e10fa6a

php 端：

 $imgAdr = "圖片的伺服器地址";

$img =  file_get_contents($imgAdr);

$imgMd5 = md5($img);

var_dump($imgMd5);  //b281bf08824cfc0668b42cf96e10fa6a