This must be terribly well known, but I can't find anything relevant. Strings in php cant have the characters < and > in them in the wamp environment. It all works fine in the live server. e g under wamp
$teststring = 'aaa<bbb';
echo $teststring;
produces aaa.
I want to edit html files using str_replace() and preg_replace(). I guess I have to modify the php set up but I don't know how.
I assume it doesn't have anything to do with WAMP, and I'm not even sure I understand the situation fully, because you say "Strings in php cant have the characters <> in them in the wamp environment" (I interpret it as: It doesn't work with WAMP) and then directly afterwards "It all works fine in the live server. e g under wamp" (I interpret it as: It does work with WAMP).
But I believe the problem is just that you are adding stray unencoded <'s into the HTML output. Think about what would happen normally:
<?php echo "I can write <em>emphasized</em> text!"; ?>
...would result in:
I can write emphasized text!
...and not:
I can write <em>emphasized</em> text!
Because you can output HTML from PHP, and the browser will read it as it would read any static HTML page. Now if you just include a random <, it will be interpreted as HTML as said in the comments and will not be valid.
So, in order to have a literal < shown in the browser, it has to be encoded as HTML entity, in this case <, e.g. 3 < 4 instead of 3 < 4. This can be automatically done using the function htmlentities. For example:
<?php echo htmlentities("This is a string with < and > and & and other stuff like this which has to be encoded."); ?>