echo substr("Hello world",6);

// Positive numbers:
echo substr("Hello world",10)."<br>"; //d
echo substr("Hello world",1)."<br>";  //ello world
echo substr("Hello world",3)."<br>";  //lo world
echo substr("Hello world",7)."<br>"; //orld
echo "<br>";

// Negative numbers:
echo substr("Hello world",-1)."<br>"; //d
echo substr("Hello world",-10)."<br>"; //ello world
echo substr("Hello world",-8)."<br>"; //lo world
echo substr("Hello world",-4)."<br>"; //orld

// Positive numbers:
echo substr("Hello world",0,10)."<br>"; //Hello worl
echo substr("Hello world",1,8)."<br>";  //ello wor
echo substr("Hello world",0,5)."<br>";  //Hello
echo substr("Hello world",6,6)."<br>";  //world
echo "<br>";

// Negative numbers:
echo substr("Hello world",0,-1)."<br>"; //Hello worl
echo substr("Hello world",-10,-2)."<br>"; //ello wor
echo substr("Hello world",0,-6)."<br>";  //Hello