nl2br();// n to
addslashes(); stripslashes();//When operating on the database, escape special characters
chop();//Remove the spaces on the right side of the string
trim();//Remove all spaces in the string
ltrim();//Remove the spaces on the left side of the string
htmlspecialchars();//Convert '$','"', '<','>' are corresponding html entities
htmlentities();//Convert all html tags to corresponding html entities
array explode(string separator, string str);// Split string
string implode(string separator, array arr);//Connect string
strtoupper(); strtolower();//Convert case
ucfirst();//Only Convert the first character to uppercase
ucwords();//Convert the first letter of each word to uppercase