Example sniff to find a string with specific content

SearchStringSniff.php

<?php
/**
 * Generic_Sniffs_Strings_SearchStringSniff.
 *
 * PHP version 5
 *
 * @category  PHP
 * @package   PHP_CodeSniffer
 * @author    Greg Sherwood <gsherwood@squiz.net>
 * @copyright 2006-2015 Squiz Pty Ltd (ABN 77 084 670 600)
 * @license   https://github.com/squizlabs/PHP_CodeSniffer/blob/master/licence.txt BSD Licence
 * @link      http://pear.php.net/package/PHP_CodeSniffer
 */

/**
 * Generic_Sniffs_Strings_SearchStringSniff.
 *
 * Searches for a specific string and does something with it.
 *
 * @category  PHP
 * @package   PHP_CodeSniffer
 * @author    Greg Sherwood <gsherwood@squiz.net>
 * @copyright 2006-2015 Squiz Pty Ltd (ABN 77 084 670 600)
 * @license   https://github.com/squizlabs/PHP_CodeSniffer/blob/master/licence.txt BSD Licence
 * @version   Release: @package_version@
 * @link      http://pear.php.net/package/PHP_CodeSniffer
 */
class Generic_Sniffs_Strings_SearchStringSniff implements PHP_CodeSniffer_Sniff
{

    /**
     * The string content to search for.
     *
     * @var string
     */
    public $searchString = 'foo';


    /**
     * Returns an array of tokens this test wants to listen for.
     *
     * @return array
     */
    public function register()
    {
        return array(T_CONSTANT_ENCAPSED_STRING);

    }//end register()


    /**
     * Processes this test, when one of its tokens is encountered.
     *
     * @param PHP_CodeSniffer_File $phpcsFile The file being scanned.
     * @param int                  $stackPtr  The position of the current token in
     *                                        the stack passed in $tokens.
     *
     * @return void
     */
    public function process(PHP_CodeSniffer_File $phpcsFile, $stackPtr)
    {
        $tokens = $phpcsFile->getTokens();

        $stringContent = trim($tokens[$stackPtr]['content'], '\'" ');
        if (strtolower($stringContent) !== $this->searchString) {
            return;
        }

        // The rest of your code here.
        $error = "You can't have a string with \"%s\" as the content";
        $data  = array($stringContent);
        $phpcsFile->addError($error, $stackPtr, 'Found', $data);

    }//end process()


}//end class

Usage

When running that sniff against this code:

<?php

echo "foo";
echo 'Foo';

You get these errors:

FILE: temp.php
----------------------------------------------------------------------
FOUND 2 ERRORS AFFECTING 2 LINES
----------------------------------------------------------------------
 3 | ERROR | [ ] You can't have a string with "foo" as the content
 4 | ERROR | [ ] You can't have a string with "Foo" as the content
----------------------------------------------------------------------

As this is a public sniff var, you can override it in a ruleset.xml file:

<rule ref="Generic.Strings.SearchString">
  <properties>
    <property name="searchString" value="bar"/>
  </properties>
</rule>