PHP substr

Summary: in this tutorial, you’ll learn how to use the PHP substr() function to extract a substring from a string.

Introduction to the PHP substr() function

The substr() function accepts a string and returns a substring from the string.

Here’s the syntax of the substr() function:

substr ( string $string , int $offset , int|null $length = null ) : string
Code language: PHP (php)

In this syntax:

  • $string is the input string.
  • $offset is the position at which the function begins to extract the substring.
  • $length is the number of characters to include in the substring. If you omit the $length argument, the substr() function will extract a substring from the $offset to the end of the $string. If the $length is 0, false, or null, the substr() function returns an empty string.

PHP substr() function examples

Let’s take some examples of using the substr() function.

1) Simple PHP substr() function example

The following example uses the substr() function to extract the first three characters from a string:

<?php $s = 'PHP substring'; $result = substr($s, 0, 3); echo $result;// PHP
Code language: HTML, XML (xml)

In this example, the substr() function extract the first 3 characters from the 'PHP substring' string starting at the index 0.

PHP substr() function

2) Using PHP substr() function with the default length argument

The following example uses the substr() function to extract a substring from the 'PHP substring' string starting from the index 4 to the end of the string:

<?php $s = 'PHP substring'; $result = substr($s, 4); echo $result; // substring
Code language: HTML, XML (xml)

In this example, we omit the $length argument. Therefore, the substr() returns a substring, starting at index 4 to the end of the input string.

PHP substr() function with default length

PHP substr() function with negative offset

The $offset argument can be a negative number. If the $offset is negative, the substr() function returns a substring that starts at the offset character from the end of the string. The last character in the input string has an index of -1.

The following example illustrates how to use the substr() function with negative offset:

<?php $s = 'PHP is cool'; $result = substr($s, -4); echo $result; // cool
Code language: HTML, XML (xml)

In this example, the substr() returns a substring that at 4th character from the end of the string.

The following picture illustrates how the substr() function works in the above example:

PHP substr() function with negative offset

PHP substr() function with negative length

Like the $offset argument, the $length argument can be negative. If you pass a negative number to the $length argument, the substr() function will omit a $length number of characters in the returned substring.

The following example illustrates how to use the substr() with a negative $offset and $length arguments:

<?php $s = 'PHP is cool'; $result = substr($s, -7, -5); echo $result; // is
Code language: HTML, XML (xml)

The following picture illustrates how the above example works:

PHP substr() function with negative length

The PHP mb_substr() function

See the following example:

<?php $message = 'adiós'; $result = substr($message, 3, 1); echo $result;
Code language: HTML, XML (xml)

This example attempts to extract a substring with one character in the $message string starting at index 3. However, it shows nothing in the output.

The reason is that the $message string contains a non-ASCII character. Therefore, the substr() function doesn’t work correctly.

To extract a substring from a string that contains a non-ASCII character, you use the mb_substr() function. The mb_substr() function is like the substr() function except that it has an additional encoding argument:

mb_substr ( string $string , int $start , int|null $length = null , string|null $encoding = null ) : string
Code language: PHP (php)

The following example uses the mb_substr() function to extract a substring from a string with non-ASCII code:

<?php $message = 'adiós'; $result = mb_substr($message, 3, 1); echo $result;
Code language: HTML, XML (xml)

Output:

ó

PHP substr helper function

The following defines a helper function that uses the mb_substr() function to extract a substring from a string:

<?php function substring($string, $start, $length = null) { return mb_substr($string, $start, $length, 'UTF-8'); }
Code language: HTML, XML (xml)

Summary

  • Use the PHP substr() function to extract a substring from a string.
  • Use the negative offset to extract a substring from the end of the string. The last character in the input string has an index of -1.
  • Use the negative length to omit a length number of characters in the returned substring.
  • Use the PHP mb_substr() function to extract a substring from a string with non-ASCII characters.
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