本文同时发表于https://github.com/zhangyachen/zhangyachen.github.io/issues/9
核心代码如下：
/* {{{ php_trim() * mode 1 : trim left * mode 2 : trim right * mode 3 : trim left and right * what indicates which chars are to be trimmed. NULL->default (‘ \t\n\r\v\0‘) */PHPAPI char *php_trim(char *c, int len, char *what, int what_len, zval *return_value, int mode TSRMLS_DC){ ???register int i; ???int trimmed = 0; ???char mask[256]; ???if (what) { ???????php_charmask((unsigned char*)what, what_len, mask TSRMLS_CC); ???} else { ???????php_charmask((unsigned char*)" \n\r\t\v\0", 6, mask TSRMLS_CC); ???} ???????//从左开始 ???if (mode & 1) { ???????for (i = 0; i < len; i++) { ???????????if (mask[(unsigned char)c[i]]) { ???????//该位置有第二个参数对应的值 ???????????????trimmed++; ???????????} else { ???????????????break; ???????????} ???????} ???????len -= trimmed; ???????c += trimmed; ???} ???if (mode & 2) { ???????for (i = len - 1; i >= 0; i--) { ???????????if (mask[(unsigned char)c[i]]) { ???????????????len--; ???????????} else { ???????????????break; ???????????} ???????} ???} ???if (return_value) { ???????????????//把c指针现在指向的位置以后的len个字符返回 ???????RETVAL_STRINGL(c, len, 1); ???} else { ???????return estrndup(c, len); ???} ???return "";}可以看出，在php_trim函数内部调用了php_charmask函数
/* {{{ php_charmask * Fills a 256-byte bytemask with input. You can specify a range like ‘a..z‘, * it needs to be incrementing. * Returns: FAILURE/SUCCESS whether the input was correct (i.e. no range errors) */static inline int php_charmask(unsigned char *input, int len, char *mask TSRMLS_DC){ ???unsigned char *end; ???unsigned char c; ???int result = SUCCESS; ???memset(mask, 0, 256); ?????//初始化一个长度为256的hash表 ???for (end = input+len; input < end; input++) { ???????c=*input; ???????if ((input+3 < end) && input[1] == ‘.‘ && input[2] == ‘.‘ ???????????????&& input[3] >= c) { ???????????memset(mask+c, 1, input[3] - c + 1); ???????????input+=3; ???????} else if ((input+1 < end) && input[0] == ‘.‘ && input[1] == ‘.‘) { ???????????/* Error, try to be as helpful as possible: ??????????????(a range ending/starting with ‘.‘ won‘t be captured here) */ ???????????if (end-len >= input) { /* there was no ‘left‘ char */ ???????????????php_error_docref(NULL TSRMLS_CC, E_WARNING, "Invalid ‘..‘-range, no character to the left of ‘..‘"); ???????????????result = FAILURE; ???????????????continue; ???????????} ???????????if (input+2 >= end) { /* there is no ‘right‘ char */ ???????????????php_error_docref(NULL TSRMLS_CC, E_WARNING, "Invalid ‘..‘-range, no character to the right of ‘..‘"); ???????????????result = FAILURE; ???????????????continue; ???????????} ???????????if (input[-1] > input[2]) { /* wrong order */ ???????????????php_error_docref(NULL TSRMLS_CC, E_WARNING, "Invalid ‘..‘-range, ‘..‘-range needs to be incrementing"); ???????????????result = FAILURE; ???????????????continue; ???????????} ???????????/* FIXME: better error (a..b..c is the only left possibility?) */ ???????????php_error_docref(NULL TSRMLS_CC, E_WARNING, "Invalid ‘..‘-range"); ???????????result = FAILURE; ???????????continue; ???????} else { ???????????????????????//对应的位置为1 ???????????mask[c]=1; ???????} ???} ???return result;}可以看出trim函数的逻辑：
1 声明一个长度为256的hash表。
2 将character_mask中每个字节转化为ascii码，将hash表中ascii码对应key的value设置为1。
3 从头部遍历str中每个字节，若遍历到字节对应的ascii码在hash表中存在，则str长度位置减1；若不存在，就中断循环。
4 从尾部遍历str中每个字节，逻辑同3。
案例分析：
trim("广东省","省")导致乱码
首先获得"广东省"的十六进制表示
<?php// 文件utf-8编码// 获取字符串编码function get_hex_str($str) { ???$hex_str = ‘‘; ???for ($i = 0; $i < strlen($str); $i ++) { ???????$ord = ord($str[$i]);//转换为ascii码 ???????$hex_str .= sprintf("\x%02X", $ord);//以十六进制输出,2为指定的输出字段的宽度.如果位数小于2,则左端补0 ???} ???return $hex_str;}$str = "广东省";
printf("str[%s] hex_str[%s]\n", \(str, get_hex_str(\)str));
\(str = trim(\)str, "省");
printf("str[%s] hex_str[%s]\n", \(str, get_hex_str(\)str));
输出：
str[广东省] hex_str[\xE5\xB9\xBF\xE4\xB8\x9C\xE7\x9C\x81]
str[广hex_str[\xE5\xB9\xBF\xE4\xB8]
utf-8编码下汉字对应三个字节，“东”的编码为e4 b8 9c，“省”的编码为e7 9c 81。
trim("广东省", "省"); 函数处理时不是以我们看到的中文字符为一个单位，而是以字节为单位。
相等于从e5 b9 bf e4 b8 9c e7 9c 81开头和结尾去掉包含在e7 9c 81的字节，这样“东”的第三个字节就会被切掉，就会有上述的输出了。
如果想将中文字符串中部分字符去掉，建议使用str_replace。