A useful function for splitting ical content into 75-octet lines, taking multibyte characters into account. See: http://www.ietf.org/rfc/rfc2445.txt, section 4.1

ical-RFC-2445-4.1.php

<?php
mb_internal_encoding("UTF-8");
$desc = <<<TEXT
<p>Lines of text SHOULD NOT be longer than 75 octets, (och hör på den) excluding the line break. Long content lines SHOULD be split into a multiple line representations using a line "folding" technique.</p>
   
   That is, a long line can be split between any two characters by inserting a CRLF
   immediately followed by a single linear white space character (i.e.,
   SPACE, <b>US-ASCII</b> decimal 32 or HTAB, US-ASCII decimal 9). Any sequence
   of CRLF followed immediately by a single linear white space character
   is ignored (i.e., removed) when processing the content type.
TEXT;

/**
 * Apply folding compliant with RFC 5545
 * See https://www.rfc-editor.org/rfc/rfc5545#section-3.1
 *
 * @param   string  $preamble   The property name, e.g. DESCRIPTION
 * @param   string  $value      The value for the property, e.g. a very long string
 * @param   bool    $strip_tags Strip HTML tags from the value
 *
 * @return  string              Returns the folded string without the property name
 */
function ical_split($preamble, $value, $strip_tags=true)
{
    $value = trim($value);
    $value = preg_replace('/[\r\n]+/', ' ', $value);
    $value = preg_replace('/\s{2,}/', ' ', $value);

    if ($strip_tags) {
        $value = strip_tags($value);
    }

    $value = $preamble . ':' . $value;
    $offset = 0;
    $chunkSize = 75;
    $lines = [];
    while ($line = mb_strcut($value, $offset, $chunkSize - 1)) {
        $lines[] = $line;
        $offset += $chunkSize;
    }

    return substr(join("\r\n\t", $lines), strlen($preamble) + 1);
}

$split = ical_split('DESCRIPTION:', $desc);
print 'DESCRIPTION:' . $split;

// Test results
$lines = preg_split('/\r\n/', 'DESCRIPTION:' . $split);

print "\n\nTests\n";
foreach ($lines as $i => $line) {
  print "Line {$i}: " . strlen($line) . " octets\n";
}

print "\nAlt desc output:\n";

$split = ical_split('X-ALT-DESC:', $desc, false);
print 'X-ALT-DESC:' . $split;
print "\n\n";

output.txt

DESCRIPTION:Lines of text SHOULD NOT be longer than 75 octets, (och hör 
	å den) excluding the line break. Long content lines SHOULD be split into 
	 multiple line representations using a line "folding" technique. That is, 
	 long line can be split between any two characters by inserting a CRLF imm
	diately followed by a single linear white space character (i.e., SPACE, US
	ASCII decimal 32 or HTAB, US-ASCII decimal 9). Any sequence of CRLF follow
	d immediately by a single linear white space character is ignored (i.e., r
	moved) when processing the content type.

Tests
Line 0: 73 octets
Line 1: 75 octets
Line 2: 75 octets
Line 3: 75 octets
Line 4: 75 octets
Line 5: 75 octets
Line 6: 75 octets
Line 7: 41 octets

Alt desc output:
X-ALT-DESC:<p>Lines of text SHOULD NOT be longer than 75 octets, (och hö
	 på den) excluding the line break. Long content lines SHOULD be split int
	 a multiple line representations using a line "folding" technique.</p> Tha
	 is, a long line can be split between any two characters by inserting a CR
	F immediately followed by a single linear white space character (i.e., SPA
	E, <b>US-ASCII</b> decimal 32 or HTAB, US-ASCII decimal 9). Any sequence o
	 CRLF followed immediately by a single linear white space character is ign
	red (i.e., removed) when processing the content type.

Huh, 14 years... time flies :)
Your implementation looks nice and elegant @viavario. Stripping out tags should probably have been separate from the folding, but I added an optional param to your implementation that can be used to disable tag stripping, preserving the old behaviour.