要理解两者的差别，先看两个文档说明：
1. 简介：

    html_entity_decode — Convert all HTML entities to their applicable characters
    htmlentities — Convert all applicable characters to HTML entities
    htmlspecialchars_decode — Convert special HTML entities back to characters
    htmlspecialchars - Convert special characters to HTML entities

2. 文档：

    htmlentities：

    This function is identical to htmlspecialchars() in all ways, * except with htmlentities(), all characters which have HTML character entity equivalents are translated into these entities.*

    htmlspecialchars ：

    If you require all input substrings that have associated named entities to be translated, use htmlentities() instead.

说明：

htmlspecialchars 只转换（& < > ’ ” ）这几个字符

而htmlentities 转换所有含有对应“html实体”的特殊字符，比如货币表示符号欧元英镑等、版权符号等。