[php] 특수문자 -> 엔티티, 엔티티 -> 특수문자
html_entity_decode(변수) : entities -> HTML
htmlentities(변수) : HTML -> entities
htmlspecialchars(변수) : 특수문자 -> HTML
html_entity_decode(변수) : entities -> HTML
htmlentities(변수) : HTML -> entities
htmlspecialchars(변수) : 특수문자 -> HTML