PHP strpos Function: Check if a String Contains Another String

The PHP strpos() function checks whether a string is contained within another string – for example checking whether a certain word appears in a sentence, and what position it appears in.  It is a great companion to the PHP substr() and str_replace() functions for processing text data.

Syntax

strpos ( $haystack , $needle [, $offset = 0 ] )

Note that:

• $haystack is the string (text or series of characters) we want to search within
• $needle is the string search term we are looking for
• $offset Is optional and defines where the search should start
  • If it is positive the search will start after that number of characters from the beginning of $haystack
  • If it is negative the search will start after that number of characters from the end of $haystack
• Returns an integer representing the position of $needle within $haystack
  • The position will start at 0 – needle is at position 0 if $haystack immediately starts with $needle
  • If $needle is not found in $haystack strpos() will return FALSE
    • This is very important if using boolean logic and if statements to check whether the string was found
    • You must check that the value returned is explicitly not equal to FALSE using === to make sure the correct action is taken

Examples

As an example we can check whether a string describing a car contains keywords:

$vehicle_description = "big red shiny car";
$car_is_red = strpos($vehicle_description, "red"); // 4
$car_is_blue = strpos($vehicle_description, "blue"); // FALSE
$car_is_big = strpos($vehicle_description, "big"); // 0
$car_is_big_ignore_first_word = strpos($vehicle_description, "big", 3); // FALSE

• $car_is_red returns 4 as the word “red” appears starting at the 4th index (position) in $vehicle_description
• $car_is_blue returns FALSE as the word “blue” does NOT appear anywhere in $vehicle_description
• $car_is_big returns 0 as the word “big” appears at the very beginning of $vehicle_description – which is index 0
  • In programming, 0 and FALSE are often treated as indicating a false value
  • This can’t be done when checking the results that return an index! As indexes start counting at 0 (zero), zero represents a positive result
  • If checking the result from str_pos() using an if statement, you must check explicitly whether the result is equal to FALSE to ensure the correct action is taken

    $vehicle_description = "big red shiny car"; 
    $car_is_blue = strpos($vehicle_description, "blue"); 
    if($car_is_blue === FALSE){ 
        echo "The car is not blue"; 
    }

• To demonstrate, we use an if statement to check the presence of “blue” in $vehicle_description, and then print some text to the browser with information based on the outcome.
• Notice we use the === operator to check both value and type so that a return of 0 does not incorrectly indicate that the string was not found

Conclusion

Knowing whether a string of text contains another string – a word or other text value – is imperative for parsing and understanding user input, or extracting useful information from database queries.

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To view the official documentation for the PHP strpos() function:

https://www.php.net/manual/en/function.strpos.php