PHP strpos() Function: locating a substring within a string

Summary: in this tutorial, you’ll learn how to use the PHP strpos() function to get the index of the first occurrence of a substring in a string.

Introduction to the PHP strpos() function #

The PHP strpos() function returns the index of the first occurrence of a substring within a string.

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

strpos ( string $haystack , string $needle , int $offset = 0 ) : int|false

The strpos() function has the following parameters:

The $haystack is a string to search in.
The $needle is a string value to search for.
The $offset is an integer that represents the index at which the strpos() function starts the search. The $offset defaults to 0.

The $offset can be positive or negative. If $offset is positive, the strpos() function starts the search at $offset number of characters to the end of the string.

If the $offset is negative, the strpos() function starts at the $offset number of characters to the beginning of the string.

If the strpos() doesn’t find the $needle in the $haystack, it returns false.

PHP strpos() function examples #

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

1) Using PHP strpos() function to search for a substring example #

The following example uses the strpos() function to search for the substring 'to' in the string 'To do or not to do':

<?php

$str = 'To do or not to do';
$position = strpos($str, 'do');

echo $position; // 3

Try it

Output:

2) Using PHP strpos() function to search for a substring with an offset example #

The following example uses the strpos() function to search for the substring 'do' in the string 'To do or not to do' start from the index 5:

<?php

$str = 'To do or not to do';
$position = strpos($str, 'do', 4);

echo $position;

Try it

Output:

In this example, the strpos() returns the index of the second occurrence of the 'do' substring because it starts the search at index 4.

3) Using PHP strpos() function gotchas #

The following example uses the strpos() function to search for the substring 'To' in the string 'To do or not to do':

<?php

$str = 'To do or not to do';
$position = strpos($str, 'To');

if ($position) {
    echo $position;
} else {
    echo 'Not found';
}

Try it

Output:

Not found

The string 'To' locates at the beginning of the string; therefore, the strpos() function returns 0. However, PHP evaluates 0 as false. So you see the 'not found' message in the output.

To avoid this issue, you should always compare the result of the strpos() function with true or false using the === or !== operator. For example:

<?php

$str = 'To do or not to do';
$position = strpos($str, 'To');

if ($position !== false) {
    echo $position; // 0
} else {
    echo 'Not found';
}

Try it

Output:

PHP stripos() function #

The stripos() function is like the strpos() function except that it searches for the substring case-insentively. The following example returns false:

<?php

$str = 'PHP is cool';
$position = strpos($str, 'php');

var_dump($position); // bool(false)

Try it

Output:

bool(false)

To search for a substring case-insensitively, you use the stripos() function:

<?php

$str = 'PHP is cool';
$position = stripos($str, 'php');

var_dump($position); // 0

Try it

Summary #

Use the strpos() function to return the index of the first occurrence of a substring in a string.
Use the stripos() function to search for the substring case-insensitively.

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