<?php
 
	$texto  = "Olá, car#oa# cliente!\n";
	$texto .= "Seja bem-vind#oa# à nossa central de atendimento para #hm#!\n";
 
	$placeholders = array( '#oa#','#hm#'     /*, etc */ );
	$masculino    = array( 'o'   ,'homens'   /*, etc */ );
	$feminino     = array( 'a'   ,'mulheres' /*, etc */ );
 
	$sexo = 'masculino';
	echo str_replace( $placeholders, $sexo=='feminino'?$feminino:$masculino, $texto );
 
	$sexo = 'feminino';
	echo str_replace( $placeholders, $sexo=='feminino'?$feminino:$masculino, $texto );
?>

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