The Strpos Trap

This week I found myself again using strpos as an example of the difference between the == and === operators in PHP. The strpos function tells us where a string occurs within another string, by returning the character offset. If the string isn't found, then the function returns false.

The problem arises because if we simply check that there's a positive return value from the function, we'll miss the case where the string appears at the start of the other string since the character offset will be zero:

$tagline = "PHP Made Easy";
if(strpos(strtolower($tagline), 'php')) {
    echo 'php tagline: ' . $tagline;
}

This code won't find the "if" to be true, since strpos returns zero and PHP will type juggle that to be false (debates about dynamically typed languages can be saved for another day)

Correct Use of ===

This is a great example of where the === operator really helps, as it can tell the difference between false and zero:

$tagline = "PHP Made Easy";
if(false !== strpos(strtolower($tagline), 'php')) {
    echo 'php tagline: ' . $tagline;
}

This time, we'll correctly get into the "if" even if the string appears at the start of the other string.

Not a particularly complex example but I think one that is simple enough to serve as a reminder as to what that extra = is for!

2 thoughts on “The Strpos Trap

  1. Type juggling can be quite tricky and lead to unexpected results as you did perfectly describe in your blog post. One should always check the manual to see the different type of return values and react accordingly.

    Another pro tip: write a wrapper function for that behaviour and call that in your code :)

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