Dennis

The QR Code Tag plugin creates QRCodes for your blog.
Features:
<ul>
<li>Choose your QR Code generator: Google Chart API (online connection required) or QR Code Lib (included)</li>
<li>Uses cURL if <code>allow_url_fopen</code> is disabled (Google Chart API) </li>
<li>GIF, PNG or JPEG image output</li>
<li>All QR Code images are cached</li>
<li>Use as a Sidebar Widget</li>
<li>Use the Shortcode <code>[qrcodetag]content[/qrcodetag]</code> within your posts</li>
<li>Use the Tooltip mode <code>[qrcodetag tooltip="content"]some text[/qrcodetag]</code> within your posts</li>
<li>Use the PHP function inside your own template</li>
<li>“Best Read Mode” for optimized QR Code image size</li>
<li>Works with PHP 5.3 as well</li>
<li>Works on symlinked plugin folders</li>
<li>Available plugin admin interface languages: English, German</li>
</ul>
<h3>Demo</h3>
See <a href="http://www.spreendigital.de/blog/2009/09/18/essential-android-applications/" rel="nofollow ugc">this blog entry</a> for a tooltip mode demo. 
Move your mouse over the application links.
<h3>Acknowledgements</h3>
This WordPress QR Code Tag Plugin is partly based on <a href="https://wordpress.org/extend/plugins/super-cool-qrcode/" rel="ugc">Super Cool QRCode Widget</a> by <a href="http://www.incerteza.org/blog/" rel="nofollow ugc">Matias S.</a>.
It uses:
<ul>
<li><a href="http://code.google.com/intl/de/apis/chart/" rel="nofollow ugc">Google Chart API</a> for online code creation</li>
<li>Y. Swetakes <a href="http://www.swetake.com/qr/index-e.html" rel="nofollow ugc">QR Code Library</a> for code offline creation</li>
<li>J. Zaefferers <a href="http://docs.jquery.com/Plugins/Tooltip" rel="nofollow ugc">jQuery Tooltip Plugin</a></li>
</ul>

QR Code Tag

Uncaught TypeError: $(...).tooltip is not a function

creates lots of errors with new wp

Use QR Codes (Google API or QR Code Lib) anywhere in your blog, as a Widget, Shortcode, Tooltip or with a PHP function.

Scott Reilly

Safely call a function, class method, or object method in a manner that doesn’t generate errors if those plugins cease to exist.
Various helper functions are provided that provide handy variations of this theme:
<ul>
<li><code>_sfc()</code>: Safely call a function and get its return value</li>
<li><code>_sfce()</code>: Safely call a function and echo its return value</li>
<li><code>_sfcf()</code>: Safely call a function; if it doesn’t exist, then a fallback function (if specified) is called</li>
<li><code>_sfcm()</code>: Safely call a function; if it doesn’t exist, then echo a message (if provided)</li>
</ul>
Let’s assume you had something like this in a template:
<pre><code><?php list_cities( 'Texas', 3 ); ?>
</code></pre>
If you deactivated the plugin that provided <code>list_cities()</code>, your site would generate an error when that template is accessed.
You can instead use <code>_sfc()</code>, which is provided by this plugin to call other functions, like so:
<pre><code><?php _sfc( 'list_cities', 'Texas', 3 ); ?>
</code></pre>
That will simply do nothing if the <code>list_cities()</code> function is not available.
If you’d rather display a message when the function does not exist, use <code>_sfcm()</code> instead, like so:
<pre><code><?php _sfcm( 'list_cities', 'The cities listing is temporarily disabled.', 'Texas', 3 ); ?>
</code></pre>
In this case, if <code>list_cities()</code> is not available, the text “The cities listing is temporarily disabled.” will be displayed.
If you’d rather call another function when the function does not exist, use _sfcf() instead, like so:
<pre><code><?php
 function unavailable_function_handler( $function_name ) { echo "The function $function_name is not available."; }
 _sfcf( 'nonexistent_function', 'unavailable_function_handler' );
?>
</code></pre>
In the event you want to safely call a function and echo its value, you can use <code>_sfce()</code> like so:
<pre><code><?php _sfce( 'largest_city', 'Tx' ); ?>
</code></pre>
Which is roughly equivalent to doing :
<pre><code><?php if function_exists( 'largest_city' ) { echo largest_city( 'Tx' ); } ?>
</code></pre>
<h4>Filter invocation method</h4>
To further prevent issues in your code should this plugin itself become deactivated, you can use indirect filter invocation to call the plugin functions. Each function has an associated filter with the same name as the function. Simply use <code>apply_filters()</code> to invoke that function instead of calling the function directly.
E.g. instead of:
<pre><code><?php _sfce( 'some_plugin_function_that_echoes', 'argument' ); ?>
</code></pre>
Do:
<pre><code><?php apply_filters( '_sfce', 'some_plugin_function_that_echoes', 'argument' ); ?>
</code></pre>
If you’re relying on the return value of a function and this plugin gets deactivated, note that the <code>apply_filters()</code> call will return the name of the function you intended to call, so you should check the return value to ensure the function got called.
Instead of:
<pre><code><?php $x = _sfc( 'some_plugin_function', 'argument' ); ?>
</code></pre>
Do:
<pre><code><?php
 $x = apply_filters( '_sfcq', 'some_plugin_function', 'argument' );
 if ( $x !== 'some_plugin_function' ) {
 // Work with the value of $x here.
 } else {
 // The Safe Function Call plugin isn't active.
 $x = 0; // Maybe set the variable to something that makes sense in this scenario.
 }
?>
</code></pre>
Links: <a href="https://coffee2code.com/wp-plugins/safe-function-call/" rel="nofollow ugc">Plugin Homepage</a> | <a href="https://wordpress.org/plugins/safe-function-call/" rel="ugc">Plugin Directory Page</a> | <a href="https://github.com/coffee2code/safe-function-call/" rel="nofollow ugc">GitHub</a> | <a href="https://coffee2code.com" rel="nofollow ugc">Author Homepage</a>
<h3>Developer Documentation</h3>
Developer documentation can be found in <a href="https://github.com/coffee2code/safe-function-call/blob/master/DEVELOPER-DOCS.md" rel="nofollow ugc">DEVELOPER-DOCS.md</a>. That documentation covers the template tags and hooks provided by the plugin.
As an overview, these are the template tags provided by the plugin:
<ul>
<li><code>_sfc()</code> : Safely call a function and get its return value.</li>
<li><code>_sfce()</code> : Safely call a function and echo its sanitized return value.</li>
<li><code>_sfcf()</code> : Safely call a function; if it doesn’t exist, then a fallback function (if specified) is called.</li>
<li><code>_sfcm()</code> : Safely call a function; if it doesn’t exist, then echo a message (if provided).</li>
</ul>
These are the hooks provided by the plugin. They are intended for filter invocation usage rather than typical content filtering.
<ul>
<li><code>_sfc</code> : Filter to safely invoke <code>_sfc()</code> in such a way that if the plugin were deactivated or deleted, then your calls to the function won’t cause errors in your site.</li>
<li><code>_sfce</code> : Filter to safely invoke <code>_sfce()</code> in such a way that if the plugin were deactivated or deleted, then your calls to the function won’t cause errors in your site.</li>
<li><code>_sfcf</code> : Filter to safely invoke <code>_sfcf()</code> in such a way that if the plugin were deactivated or deleted, then your calls to the function won’t cause errors in your site.</li>
<li><code>_sfcm</code> : Filter to safely invoke <code>_sfcm()</code> in such a way that if the plugin were deactivated or deleted, then your calls to the function won’t cause errors in your site.</li>
</ul>

Safe Function Call

Safely and easily call functions that may not be available (such as those provided by a plugin that gets deactivated)

QR Code Tag vs Safe Function Call