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<?php $string = 'III.OOO.OOO.S'; $search = 'OOO'; $replace = 'B'; //replace ONLY FIRST occurance of "OOO" with "B" echo substr_replace($string,$replace,0,strlen($search)); //$string => B.OOO.OOO.S echo "\n---\n"; //replace ONLY LAST occurance of "OOOO" with "B" echo substr_replace($string,$replace,strrpos($string,$search),strlen($search)); //$string => OOO.OOO.B.S echo "\n---\n"; //replace ONLY LAST occurance of "OOOO" with "B" echo strrev(implode(strrev($replace),explode(strrev($search),strrev($string),2))); //$string => OOO.OOO.B.S

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