Bits & Bytes

How do you find a substring within a string? In PHP, the answer is not quite as straightforward as it should be. But, it’s still easy to do–you just need to be aware of a couple things.

Suppose we have the string “ABCDEFG”. There are two cases we want to look at: when we want to find “DEF” in the middle of the string, and when we want to find “ABC” at the beginning.

The function to find a substring is called strpos(). It can take up to three parameters:

strpos($haystack, $needle, $offset);

1. $haystack: (required) the full string we want to search within
2. $needle: (required) the substring we are searching for
3. $offset: (optional) if we want the function to start looking for the substring at a position which is not at the beginning of the $haystack

When we call strpos() on the string, it will return the position of the substring as an integer (starting from 0), if it finds it. If it doesn’t find the substring, it will return false (which is 0 in PHP). So, when we search for “DEF”, the function will return 3 which is the position of the “D” (base 0). However, when we search for “ABC”, strpos() will return 0 because it found the substring at position 0.

So, we have to construct our code in this way to check the return value from the function:

$ret = strpos("ABCDEFG", "ABC");

if($ret === false) {
    echo("We did not find the substring.");
} else {
    echo("We found the substring!");
}

Here are some sample cases:

strpos(“ABCDEFG”, “ABC”); –> returns integer 0, substring found

strpos(“ABCDEFG”, “DEF”); –> returns integer 3, substring found

strpos(“ABCDEFG”, “XYZ”); –> returns a 0 = to PHP’s false keyword. substring not found

In the above code, the operator === checks whether two values are identical, which means whether the two values are of the same PHP type. The strpos() function will return a 0 that is equivalent to the false keyword in PHP if it does not find the substring. It will return an integer 0 if it finds the substring starting at position 0.  So, we use the operator which checks whether the two values are identical, and if a 0 was returned, whether the function meant that 0 to be false or to be the position of the substring it found.

Tags: algorithm, computer, find, php, programming, search, string, substring, tutorial, web

By: Jenna Hall

This entry was posted on Tuesday, December 28th, 2010 at 12:28 pm and is filed under PHP. You can follow any responses to this entry through the RSS 2.0 feed.Both comments and pings are currently closed.