How do I check if a string contains a specific word?

Consider:

$a = 'How are you?';

if ($a contains 'are')
    echo 'true';

Suppose I have the code above, what is the correct way to write the statement if ($a contains 'are')?

You can use the strpos() function which is used to find the occurrence of one string inside another one:

$a = 'How are you?';

if (strpos($a, 'are') !== false) {
    echo 'true';
}

Note that the use of !== false is deliberate (neither != false nor === true will return the desired result); strpos() returns either the offset at which the needle string begins in the haystack string, or the boolean false if the needle isn't found. Since 0 is a valid offset and 0 is "falsey", we can't use simpler constructs like !strpos($a, 'are').

Now with PHP 8 you can do this using str_contains:

if (str_contains('How are you', 'are')) { 
    echo 'true';
}

RFC

You could use regular expressions as it's better for word matching compared to strpos, as mentioned by other users. A strpos check for are will also return true for strings such as: fare, care, stare, etc. These unintended matches can simply be avoided in regular expression by using word boundaries.

A simple match for are could look something like this:

$a = 'How are you?';

if (preg_match('/\bare\b/', $a)) {
    echo 'true';
}

On the performance side, strpos is about three times faster. When I did one million compares at once, it took preg_match 1.5 seconds to finish and for strpos it took 0.5 seconds.

Edit: In order to search any part of the string, not just word by word, I would recommend using a regular expression like

$a = 'How are you?';
$search = 'are y';
if(preg_match("/{$search}/i", $a)) {
    echo 'true';
}

The i at the end of regular expression changes regular expression to be case-insensitive, if you do not want that, you can leave it out.

Now, this can be quite problematic in some cases as the $search string isn't sanitized in any way, I mean, it might not pass the check in some cases as if $search is a user input they can add some string that might behave like some different regular expression...

Also, here's a great tool for testing and seeing explanations of various regular expressions Regex101

To combine both sets of functionality into a single multi-purpose function (including with selectable case sensitivity), you could use something like this:

function FindString($needle,$haystack,$i,$word)
{   // $i should be "" or "i" for case insensitive
    if (strtoupper($word)=="W")
    {   // if $word is "W" then word search instead of string in string search.
        if (preg_match("/\b{$needle}\b/{$i}", $haystack)) 
        {
            return true;
        }
    }
    else
    {
        if(preg_match("/{$needle}/{$i}", $haystack)) 
        {
            return true;
        }
    }
    return false;
    // Put quotes around true and false above to return them as strings instead of as bools/ints.
}

One more thing to take in mind, is that \b will not work in different languages other than english.

The explanation for this and the solution is taken from here:

\b represents the beginning or end of a word (Word Boundary). This regex would match apple in an apple pie, but wouldn’t match apple in pineapple, applecarts or bakeapples.

How about “café”? How can we extract the word “café” in regex? Actually, \bcafé\b wouldn’t work. Why? Because “café” contains non-ASCII character: é. \b can’t be simply used with Unicode such as समुद्र, 감사, месяц and 😉 .

When you want to extract Unicode characters, you should directly define characters which represent word boundaries.

The answer: (?<=[\s,.:;"']|^)UNICODE_WORD(?=[\s,.:;"']|$)

So in order to use the answer in PHP, you can use this function:

function contains($str, array $arr) {
    // Works in Hebrew and any other unicode characters
    // Thanks https://medium.com/@shiba1014/regex-word-boundaries-with-unicode-207794f6e7ed
    // Thanks https://www.phpliveregex.com/
    if (preg_match('/(?<=[\s,.:;"\']|^)' . $word . '(?=[\s,.:;"\']|$)/', $str)) return true;
}

And if you want to search for array of words, you can use this:

function arrayContainsWord($str, array $arr)
{
    foreach ($arr as $word) {
        // Works in Hebrew and any other unicode characters
        // Thanks https://medium.com/@shiba1014/regex-word-boundaries-with-unicode-207794f6e7ed
        // Thanks https://www.phpliveregex.com/
        if (preg_match('/(?<=[\s,.:;"\']|^)' . $word . '(?=[\s,.:;"\']|$)/', $str)) return true;
    }
    return false;
}

As of PHP 8.0.0 you can now use str_contains

<?php
    if (str_contains('abc', '')) {
        echo "Checking the existence of the empty string will always 
        return true";
    }