<?php
 
$replaces = array(
    '/a/' => 'b',
    '/b/' => 'x'
);
// primeiro troca "a" por "b", se tornando "bb"
// depois troca "b" por "x", o resultado é "xx"
echo preg_replace(array_keys($replaces), array_values($replaces), 'ab');
 
echo PHP_EOL;
 
$replaces = array(
    '/b/' => 'x',
    '/a/' => 'b'
);
// primeiro troca "b" por "x", se tornando "ax"
// depois troca "a" por "b", o resultado é "bx"
echo preg_replace(array_keys($replaces), array_values($replaces), 'ab');

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