Don't use this.x for starters, that's a reserved variable for NPCs. Use pos or positions instead.
PHP Code:
function onCreated() {
temp.str.loadstring("logs/testlog.txt");
temp.spos = temp.str.pos("Testing123");
if (temp.spos >= 0) {
echo("found string");
echo(temp.str.substring(temp.spos - 100, temp.spos + 100));
}
for (temp.spos: temp.str.positions("Testing123")) {
echo(temp.str.substring(temp.spos - 100, temp.spos + 100));
}
}