htmlentities跟htmlspecialchars都是用于将字符串的特殊字符中转成HTML字符实体。只是htmlspecialcharsl转义的特殊字符集只有5个，
'&' (ampersand) => '&'
'"' (double quote) => '"' when ENT_NOQUOTES is not set.
''' (single quote) => ''' only when ENT_QUOTES is set.
'<' (less than) => '<'
'>' (greater than) => '>'
而htmlentities转义的特殊字符集是完整的HTML字符实体集。
对于这两个函数，php手册上都是英文做的解释，其中在htmlentities函数的说明部分有这么一段英文：
This function is identical(完全相同的) to htmlspecialchars() in all ways, except with htmlentities(), all characters which have HTML character entity equivalents(HTML
character entity equivalents译为HTML字符实体等价物) are translated into these entities.
从这句话中我们也可以看出来这两个函数虽然基本功能差不多，但是还是有细微的差别在里面的。再仔细看htmlspecialchars函数里面的一段话：
Certain characters(其实就是'&', 双引号(没有设置ENT_NOQUOTES时才会转义), 单引号(当且仅当设置ENT_QUOTES才会转义), '<', '>') have special significance in HTML, and should be represented by HTML entities if they are to preserve their meanings. This function
returns a string with some of these conversions made; the translations made are those most useful for everyday web programming. If you require all HTML character entities to be translated, use htmlentities() instead.
注：使用这两个函数时最后指明编码格式哦。
参考链接
- W3Schools.com - Symbols Entities
- 这是一个完整的字符实体参考 Webstandards.org - Symbol Entities
-
http://www.neoease.com/using-html-symbol-entities/