htmlentities()
htmlentities() is a native PHP function which converts special characters in a string to their equivalent in HTML.
htmlentities() takes the string to be encoded, as an argument. It also accepts an argument $encoding to specify what the original encoding of the string is, in particular when it is not utf-8, which is the default configuration.
There are options to control which characters shall be encoded: for example, single and double quotes may be converted or not, with ENT_QUOTES and ENT_NOQUOTES, respectively.
Finally, $double_encode makes htmlentities() skip encoding a second time any entities already set in the string.
The opposite of htmlentities() is html_entity_decode().
<?php
$str = "A 'quote' is <b>bold</b>";
echo htmlentities($str);
echo "\n\n";
echo htmlentities($str, ENT_COMPAT);
?>