php word replace , online demo https://ideone.com/94EjGy

str_replace_word.php

<?php

function str_replace_word($cari, $ganti, $kalimat)
{
    $cek = strpos($kalimat, " ");
    if ($cek != false) {
        $token = explode(" ", $kalimat);
        $map = array_map(function($val) use ($cari, $ganti)
        {
            return $val === $cari ? $ganti : $val;
        }, $token);
        return implode(" ", $map);
    }
    return $kalimat === $cari ? $ganti : $kalimat;
}

$kalimat = "nama akun jono adalah @jono123.\n";

echo $kalimat;
echo str_replace("jono", "budi", $kalimat);
echo str_replace_word("jono", "budi", $kalimat);