This is an email encrypt / decrypt extracted from TYPO3. It is useful to protecting your email against spam bots.

## email-encrypt-decrypt.phtml
<?php
$email = 'software@insanitymeetshh.net';

$emailEncode = quoteJSvalue(encryptEmail($email, '-2'));

function quoteJSvalue($value) {
    return strtr(
        json_encode((string)$value, JSON_HEX_AMP|JSON_HEX_APOS|JSON_HEX_QUOT|JSON_HEX_TAG),
        [
            '"' => '\'',
            '\\\\' => '\\u005C',
            ' ' => '\\u0020',
            '!' => '\\u0021',
            '\\t' => '\\u0009',
            '\\n' => '\\u000A',
            '\\r' => '\\u000D'
        ]
    );
}

function encryptCharcode($n, $start, $end, $offset) {
    $n = $n + $offset;
    if ($offset > 0 && $n > $end) {
        $n = $start + ($n - $end - 1);
    } elseif ($offset < 0 && $n < $start) {
        $n = $end - ($start - $n - 1);
    }
    return chr($n);
}

function encryptEmail($string, $type) {
    $out = '';
    // obfuscates using the decimal HTML entity references for each character
    if ($type === 'ascii') {
        $stringLength = strlen($string);
        for ($a = 0; $a < $stringLength; $a++) {
            $out .= '&#' . ord(substr($string, $a, 1)) . ';';
        }
    } else {
        // like str_rot13() but with a variable offset and a wider character range
        $len = strlen($string);
        $offset = (int)$type;
        for ($i = 0; $i < $len; $i++) {
            $charValue = ord($string[$i]);
            // 0-9 . , - + / :
            if ($charValue >= 43 && $charValue <= 58) {
                $out .= encryptCharcode($charValue, 43, 58, $offset);
            } elseif ($charValue >= 64 && $charValue <= 90) {
                // A-Z @
                $out .= encryptCharcode($charValue, 64, 90, $offset);
            } elseif ($charValue >= 97 && $charValue <= 122) {
                // a-z
                $out .= encryptCharcode($charValue, 97, 122, $offset);
            } else {
                $out .= $string[$i];
            }
        }
    }
    return $out;
}
?>
<html>
    <head>
        <script type="text/javascript">
            /*<![CDATA[*/
            /*_scriptCode*/

            // decrypt helper function
            function decryptCharcode(n,start,end,offset) {
                n = n + offset;
                if (offset > 0 && n > end) {
                    n = start + (n - end - 1);
                } else if (offset < 0 && n < start) {
                    n = end - (start - n - 1);
                }
                return String.fromCharCode(n);
            }

            // decrypt string
            function decryptString(enc,offset) {
                var dec = "";
                var len = enc.length;
                for(var i=0; i < len; i++) {
                    var n = enc.charCodeAt(i);
                    if (n >= 0x2B && n <= 0x3A) {
                        dec += decryptCharcode(n,0x2B,0x3A,offset);	// 0-9 . , - + / :
                    } else if (n >= 0x40 && n <= 0x5A) {
                        dec += decryptCharcode(n,0x40,0x5A,offset);	// A-Z @
                    } else if (n >= 0x61 && n <= 0x7A) {
                        dec += decryptCharcode(n,0x61,0x7A,offset);	// a-z
                    } else {
                        dec += enc.charAt(i);
                    }
                }
                return dec;
            }

            // decrypt spam-protected emails
            function linkTo_UnCryptMailto(s) {
                location.href = 'mailto:' + decryptString(s,2);
                //alert(decryptString(s,2));
            }
        /*]]>*/
        </script>
    </head>
    <body>
        <?php echo $email . '<br/>'; ?>
        <?php echo '<a href="javascript:linkTo_UnCryptMailto(' . $emailEncode . ');">' . $emailEncode . '</a>'; ?>
    </body>
</html>
	<?php
	$email = 'software@insanitymeetshh.net';

	$emailEncode = quoteJSvalue(encryptEmail($email, '-2'));

	function quoteJSvalue($value) {
	return strtr(
	json_encode((string)$value, JSON_HEX_AMP\|JSON_HEX_APOS\|JSON_HEX_QUOT\|JSON_HEX_TAG),
	[
	'"' => '\'',
	'\\\\' => '\\u005C',
	' ' => '\\u0020',
	'!' => '\\u0021',
	'\\t' => '\\u0009',
	'\\n' => '\\u000A',
	'\\r' => '\\u000D'
	]
	);
	}

	function encryptCharcode($n, $start, $end, $offset) {
	$n = $n + $offset;
	if ($offset > 0 && $n > $end) {
	$n = $start + ($n - $end - 1);
	} elseif ($offset < 0 && $n < $start) {
	$n = $end - ($start - $n - 1);
	}
	return chr($n);
	}

	function encryptEmail($string, $type) {
	$out = '';
	// obfuscates using the decimal HTML entity references for each character
	if ($type === 'ascii') {
	$stringLength = strlen($string);
	for ($a = 0; $a < $stringLength; $a++) {
	$out .= '&#' . ord(substr($string, $a, 1)) . ';';
	}
	} else {
	// like str_rot13() but with a variable offset and a wider character range
	$len = strlen($string);
	$offset = (int)$type;
	for ($i = 0; $i < $len; $i++) {
	$charValue = ord($string[$i]);
	// 0-9 . , - + / :
	if ($charValue >= 43 && $charValue <= 58) {
	$out .= encryptCharcode($charValue, 43, 58, $offset);
	} elseif ($charValue >= 64 && $charValue <= 90) {
	// A-Z @
	$out .= encryptCharcode($charValue, 64, 90, $offset);
	} elseif ($charValue >= 97 && $charValue <= 122) {
	// a-z
	$out .= encryptCharcode($charValue, 97, 122, $offset);
	} else {
	$out .= $string[$i];
	}
	}
	}
	return $out;
	}
	?>
	<html>
	<head>
	<script type="text/javascript">
	/<![CDATA[/
	/_scriptCode/

	// decrypt helper function
	function decryptCharcode(n,start,end,offset) {
	n = n + offset;
	if (offset > 0 && n > end) {
	n = start + (n - end - 1);
	} else if (offset < 0 && n < start) {
	n = end - (start - n - 1);
	}
	return String.fromCharCode(n);
	}

	// decrypt string
	function decryptString(enc,offset) {
	var dec = "";
	var len = enc.length;
	for(var i=0; i < len; i++) {
	var n = enc.charCodeAt(i);
	if (n >= 0x2B && n <= 0x3A) {
	dec += decryptCharcode(n,0x2B,0x3A,offset); // 0-9 . , - + / :
	} else if (n >= 0x40 && n <= 0x5A) {
	dec += decryptCharcode(n,0x40,0x5A,offset); // A-Z @
	} else if (n >= 0x61 && n <= 0x7A) {
	dec += decryptCharcode(n,0x61,0x7A,offset); // a-z
	} else {
	dec += enc.charAt(i);
	}
	}
	return dec;
	}

	// decrypt spam-protected emails
	function linkTo_UnCryptMailto(s) {
	location.href = 'mailto:' + decryptString(s,2);
	//alert(decryptString(s,2));
	}
	/]]>/
	</script>
	</head>
	<body>
	<?php echo $email . '<br/>'; ?>
	<?php echo '<a href="javascript:linkTo_UnCryptMailto(' . $emailEncode . ');">' . $emailEncode . '</a>'; ?>
	</body>
	</html>