php addslashes函数最终对应的c函数为：
3244 PHPAPI char *php_addslashes_ex(char *str, int length, int *new_length, int should_free, int ignore_sybase TS RMLS_DC)
3245 {
3246 /* maximum string length, worst case situation */
3247 char *new_str;
3248 char *source, *target;
3249 char *end;
3250 int local_new_length;
3251
3252 if (!new_length) {
3253 new_length = &local_new_length;
3254 }
3255 if (!str) {
3256 *new_length = 0;
3257 return str;
3258 }
3259 new_str = (char *) safe_emalloc(2, (length ? length : (length = strlen(str))), 1);
3260 source = str;
3261 end = source + length;
3262 target = new_str;
3263
3264 if (!ignore_sybase && PG(magic_quotes_sybase)) {
3265 while (source
3266 switch (*source) {
3267 case ' ':
3268 *target++ = '\';
3269 *target++ = '0';
3270 break;
3271 case ''':
3272 *target++ = ''';
3273 *target++ = ''';
3274 break;
3275 default:
3276 *target++ = *source;
3277 break;
3278 }
3279 source++;
3280 }
3281 } else {
3282 while (source
3283 switch (*source) {
3284 case ' ':
3285 *target++ = '\';
3286 *target++ = '0';
3287 break;
3288 case ''':
3289 case '"':
3290 case '\':
3291 *target++ = '\';
3292 /* break is missing *intentionally* */
3293 default:
3294 *target++ = *source;
3295 break;
3296 }
3297
3298 source++;
3299 }
3300 }
3301
3302 *target = 0;
3303 *new_length = target - new_str;
3304 if (should_free) {
3305 STR_FREE(str);
3306 }
3307 new_str = (char *) erealloc(new_str, *new_length + 1);
3308 return new_str;
3309 }
从上面看addslashes函数处理的是单字节的字节。
但是gbk编码是双字节的。而且5c又可以做双字节的低位。
