<?php
function replaceFromPos($str, $reg, $repl, $pos = 0, $cnt = -1){
    return substr($str, 0, $pos).preg_replace($reg, $repl, substr($str, $pos, strlen($str)), $cnt);
}
$text = 'hello world! this world is great. world is big!';
$pattern = '/(world)/i';
$replacement = 'WORLD';
// заменим одно слово, начиная с 10-ой позиции
echo replaceFromPos($text, $pattern, $replacement, 10, 1) , PHP_EOL;
// заменим всё, что после 10-ой позиции
echo replaceFromPos($text, $pattern, $replacement, 10) , PHP_EOL;
// заменим первые два вхождения с нулевой позиции
echo replaceFromPos($text, $pattern, $replacement, 0, 2) , PHP_EOL;

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