Substring. In PHP programs we often need to access just a part of a string, not the entire string. Consider the word "substring": we can just access the "string" part of it.

Function arguments. The PHP substr function receives the source string, the offset index (0-based) and a length. We must pass a length, not the end index.

Substr example. To begin, we invoke the substr function twice. We first declare a string containing the letters "carrot" and then we get parts of it.

$word = "carrot";

// Part 1: get first 3 characters.
$result1 = substr($word, 0, 3);
var_dump($result1);

// Part 2: get last 3 characters.
$result2 = substr($word, -3);
var_dump($result2);
string(3) "car"
string(3) "rot"

Assign index. We can modify a string in PHP by assigning into it. When we assign a character by index, we can only place one new character there.

// Part 1: get character from string.
$input = "abc";
$second = $input[1];

var_dump($second);

// Part 2: change character in string.
$input[1] = "?";

var_dump($input);
string(1) "b"
string(3) "a?c"

Substr replace. This method changes a range of characters in the input string. So we specify a start index, and a length, and that part of the string is replaced with something else.

// The input string.
$input = "abc abc";

// Replace chars starting at index 1 and continuing for 2 chars.
$result = substr_replace($input, "??", 1, 2);
var_dump($result);
string(7) "a?? abc"

Taking substrings in PHP is common—nearly every PHP program in the real world does this. For 1-character substrings, we can use an index expression.