What potential issues can arise when changing the encoding from UTF-8 to ISO-8859-1 in PHP applications?

Changing the encoding from UTF-8 to ISO-8859-1 in PHP applications can lead to character encoding issues, as ISO-8859-1 does not support all Unicode characters. To solve this problem, you can convert the string from UTF-8 to ISO-8859-1 using the `iconv` function in PHP.

// Convert UTF-8 string to ISO-8859-1
$utf8String = "Hello, world!";
$iso8859String = iconv("UTF-8", "ISO-8859-1", $utf8String);

echo $iso8859String;