public OnPlayerText(playerid, text[])
{
    for(new x = 0; x <= strlen(text); ++x)
    {
        if(text[(x)]=='t' && text[(x+1)]=='o' && text[(x+2)]=='o' && text[(x+3)]=='l') //Look that "if" checks if the character is part of the word that you choose, and if it is really the word.
        {
            text[(x)] = 'b';
            text[(x+1)] = 'e';
            text[(x+2)] = 's';
            text[(x+3)] = 't';  //It will change "tool" for "best"
        }
    }
    return 1;
}