truncating imported rss-feeds

Use PHP string functions like substr to return a number of characters rather than word count.

The post content aaaaaaaaaaaaaaa aaaaaaaaaaaaa will be much longer than a a.

$max_length = 150;

echo substr( $string, 0, $max_length );

As a function it might look like:

function string_limit_words( $string = '', $count = 25, $after="..." ) {
    if ( strlen( $string ) <= $count ) {
        return $string;
    }

    return substr( $string, 0, $count ) . $after;
}

echo string_limit_words( "This is a lot of words", 10 );

// This is a ...

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the_excerpt() prints the output while get_the_excerpt() will return the string to allow manipulation before output.