This function is identical to htmlspecialchars() in all ways, except that all characters which have HTML character entity equivalents are translated into these entities. Like htmlspecialchars(), it takes an optional second argument which indicates what should be done with single and double quotes. ENT_COMPAT (the default) will only convert double-quotes and leave single-quotes alone. ENT_QUOTES will convert both double and single quotes, and ENT_NOQUOTES will leave both double and single quotes unconverted.
At present, the ISO-8859-1 character set is used as default. Support for the optional second argument was added in PHP 3.0.17 and PHP 4.0.3.
Like htmlspecialchars(), it takes an optional third argument which defines character set used in conversion. Support for this argument was added in PHP 4.1.0.
There is no reverse of this function. However, you can create one on your own. Here is an example of how to do this.