Why Does Loose Comparison with strpos() in PHP Lead to Unexpected Results?

Loose Interpretation of strpos() Return Values Leads to Unexpected Outcomes

In a quest to locate specific substrings within a string, there lies a pitfall that can lead to unintended results when using PHP's strpos() function. Let's unravel the mystery behind why loose checking of strpos() return values can cause confusion.

Consider the code snippet below:

if (
    strpos($grafik['data'], $ss1) != false
    && strpos($grafik['data'], $ss2) != false
    && strpos($grafik['data'], $ss1) < strpos($grafik['data'],$ss2)
)

In this code, strpos() is employed to determine the positions of substrings $ss1 and $ss2 within the $grafik['data'] string. The intent is to check for the presence of both substrings and ensure that $ss1 is ordered before $ss2.

According to the PHP manual, strpos() returns false if the substring is not found. However, it's discovered that when the substring begins at the zero position (i.e., the start of the string), strpos() returns 0. This leads to an unintended interpretation where the expression:

strpos($grafik['data'], $ss1) != false

evaluates to false, despite $ss1 being present from the zero position. The culprit is the weak equivalence operator !=, which considers 0 to be equivalent to false.

To rectify this issue, the strict equivalence operator === should be used instead:

if (
    strpos($grafik['data'], $ss1) !== false
    && strpos($grafik['data'], $ss2) !== false
    && strpos($grafik['data'], $ss1) < strpos($grafik['data'],$ss2)
)

The === operator performs a strict comparison, ensuring that 0 is not considered equivalent to false. With this correction, the code will correctly determine the presence of $ss1 and $ss2 and their relative positions.

The above is the detailed content of Why Does Loose Comparison with strpos() in PHP Lead to Unexpected Results?. For more information, please follow other related articles on the PHP Chinese website!