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English
In PHP, the strpos function is used to find the position of the first occurrence of a substring within a string. Its basic syntax is as follows:
<span><span><span class="hljs-title function_ invoke__">strpos</span></span><span>(</span><span><span class="hljs-keyword">string</span></span><span> </span><span><span class="hljs-variable">$haystack</span></span><span>, </span><span><span class="hljs-keyword">mixed</span></span><span> </span><span><span class="hljs-variable">$needle</span></span><span>, </span><span><span class="hljs-keyword">int</span></span><span> </span><span><span class="hljs-variable">$offset</span></span><span> = </span><span><span class="hljs-number">0</span></span><span>): </span><span><span class="hljs-keyword">int</span></span><span>|</span><span><span class="hljs-literal">false</span></span><span>
</span></span>Here, $haystack is the target string, $needle is the substring to search for, and $offset is an optional parameter that indicates where to start the search.
Typically, strpos returns the index (starting from 0) of the first occurrence of the substring. If the substring is not found, it returns false.
First, it’s important to understand how the strpos function behaves by default. When no offset ($offset) is provided, the function begins the search from the start of the string. Consider the following example:
<span><span><span class="hljs-variable">$haystack</span></span><span> = </span><span><span class="hljs-string">"Hello, world!"</span></span><span>;
</span><span><span class="hljs-variable">$needle</span></span><span> = </span><span><span class="hljs-string">"world"</span></span><span>;
<p></span>$position = strpos($haystack, $needle);<br>
echo $position; // Outputs 7<br>
</span>In this example, strpos returns 7, which is the position where "world" first appears in "Hello, world!".
The third parameter of the strpos function, $offset, is optional. It allows us to specify where in the target string the search should begin. For example:
<span><span><span class="hljs-variable">$haystack</span></span><span> = </span><span><span class="hljs-string">"Hello, world!"</span></span><span>;
</span><span><span class="hljs-variable">$needle</span></span><span> = </span><span><span class="hljs-string">"o"</span></span><span>;
<p></span>$position = strpos($haystack, $needle, 5);<br>
echo $position; // Outputs 8<br>
</span>In this example, strpos starts the search from index 5, so it returns the position of the second "o" in "world", which is 8.
When a negative number is passed as the $offset, the strpos function searches backward from the end of the string. Specifically, a negative offset means the search starts that many characters from the end.
For example:
<span><span><span class="hljs-variable">$haystack</span></span><span> = </span><span><span class="hljs-string">"Hello, world!"</span></span><span>;
</span><span><span class="hljs-variable">$needle</span></span><span> = </span><span><span class="hljs-string">"o"</span></span><span>;
<p></span>$position = strpos($haystack, $needle, -6);<br>
echo $position; // Outputs 8<br>
</span>In this case, -6 starts the search 6 characters before the end of the string, i.e., from "world!". strpos finds the "o" and returns its position: 8.
Let’s look at another example to understand how negative offsets are interpreted:
<span><span><span class="hljs-variable">$haystack</span></span><span> = </span><span><span class="hljs-string">"abcdef"</span></span><span>;
</span><span><span class="hljs-variable">$needle</span></span><span> = </span><span><span class="hljs-string">"c"</span></span><span>;
<p></span>$position = strpos($haystack, $needle, -2);<br>
echo $position; // Outputs 2<br>
</span>Here, the offset -2 starts the search at the character "e", the second-to-last character in "abcdef". strpos searches backward and finds "c" at position 2.
While negative offsets offer flexibility, there are a few things to keep in mind:
Offset shouldn't be too negative: If the absolute value of the offset is greater than the string length, strpos will behave as though no offset was provided and start from the beginning. For example, passing -20 when the string has only 10 characters will be equivalent to starting at position 0.
Unexpected results may occur: Negative offsets make the logic more complex. If you're unsure of the string length or search starting point, the result might not be as expected. Developers should fully understand the relationship between offset values and string length.
PHP's strpos function supports negative offset values, enabling searches from the end of a string backward. This offers greater flexibility but also requires caution—ensure that the offset’s absolute value does not exceed the string length. Understanding this mechanism helps developers perform string searches more accurately and avoid unexpected behavior.
