substr()

The substr() function returns a slice of a string — the characters between a starting position and an optional length. It's one of the most-used string tools in PHP: trimming prefixes, taking the first N characters of a label, reading a fixed-width field, or grabbing a file extension all come down to a substr() call. This page covers the syntax, every way the $start and $length arguments behave (including negatives), the edge cases that trip people up, and when to reach for mb_substr() instead.

Syntax

substr(string $string, int $offset, ?int $length = null): string

substr() returns the extracted substring. As of PHP 8.0 it always returns a string (an empty string when nothing matches); before PHP 8.0 it returned false on failure, so you may still see that in older code.

A basic example

$offset is 0, so extraction starts at the first character, and $length is 5, so five characters (Hello) are returned. Remember the offset is zero-based: position 0 is H, position 1 is e, and so on.

How the offset works

A positive offset counts from the start; a negative offset counts from the end (where -1 is the last character):

<?php
$string = "Hello World!";

echo substr($string, 6);   // World!  — from offset 6 to the end
echo "\n";
echo substr($string, -6);  // World!  — last 6 characters
echo "\n";
echo substr($string, -1);  // !       — just the last character

Omitting $length returns everything from the offset onward — handy for stripping a known prefix.

How the length works

A positive $length limits how many characters you take. A negative $length stops that many characters before the end of the string, which is great for chopping off a known suffix:

<?php
$string = "Hello World!";

echo substr($string, 0, -1);   // Hello World  — drop the trailing "!"
echo "\n";
echo substr($string, -6, 5);   // World        — start 6 from the end, take 5
echo "\n";
echo substr($string, 4, 0);    // (empty)      — length 0 returns ""

If the requested length runs past the end of the string, substr() simply returns whatever is available — it never errors for being "too long."

Practical use: a file extension

Combining substr() with strrpos() (which finds the last position of a character) is the classic way to read a file extension:

<?php
$filename = "report.final.pdf";
$ext = substr($filename, strrpos($filename, ".") + 1);

echo $ext; // pdf

Common gotchas

• Offset out of range. If $offset is greater than the string length, you get an empty string (in PHP 8+). It does not wrap around.
• Counts bytes, not characters. substr() works on bytes. For UTF-8 text with accented letters or emoji, slicing mid-character produces garbage. Use mb_substr() for multibyte-safe extraction.
• Off-by-one with length. $length is a count, not an end index. substr($s, 2, 3) returns 3 characters starting at index 2 — not the characters between index 2 and index 3.

Related functions

• strlen() — get the length of a string, often used to compute a $length.
• strpos() — find where a substring begins, to feed into substr().
• explode() — split a string into an array when you have a delimiter.
• trim() — remove whitespace (or other characters) from both ends.

Practice