infatti come dicevo, ecco il test che invece funziona:

Codice PHP:
$string 'The quick brown fox jumped quick over the lazy dog.';
                                        
$patterns = array();
                                        
$patterns[0] = '/quick/';
                                        
$patterns[1] = '/brown/';
                                        
$replacements = array();
                                        
$replacements[0] = 'xxx';
                                        
$replacements[1] = 'yyy';
                                        echo 
preg_replace($patterns$replacements$string1); 
in questo caso essendo LIMIT 1 la stringa "quick" verrà sostituita solo la prima occorrenza