Replace last word in string

You can find the last whitespace using the strrpos() function:

$variable = 'put returns between paragraphs';
$lastSpace = strrpos($variable, ' '); // 19

Then, take the two substrings (before and after the last whitespace) and wrap around the 'and':

$before = substr($variable, 0, $lastSpace); // 'put returns between'
$after = substr($variable, $lastSpace); // ' paragraphs' (note the leading whitespace)
$result = $before . ' and' . $after;

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EDIT
Although nobody wants to mess with substring indexes this is a very basic task for which PHP ships with useful functions (specificly strrpos() and substr()). Hence, there's no need to juggle arrays, reversed strings or regexes - but you can, of course :)

You can use preg_replace():

$add = 'and';
$variable = 'put returns between paragraphs';    
echo preg_replace("~\W\w+\s*$~", ' ' . $add . '\\0', $variable);

Prints:

put returns between and paragraphs

This will ignore trailing whitespaces, something @jensgram's solution doesn't. (eg: it will break if your string is $variable = 'put returns between paragraphs '. Of course you can use trim(), but why bother to waste more memory and call another function when you can do it with regex ? :-)