#!/bin/bash
s="define( 'PB_PASSWORD', 'ab57a5449b0781c91c4701ab6258655d' );"
sed "s/\(define( 'PB_PASSWORD', '\)[^']*/\1438bb1dc13ec5cbdf3b938e4fc6c3748/" <<< "$s"
# define( 'PB_PASSWORD', '438bb1dc13ec5cbdf3b938e4fc6c3748' );