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My site requires the use of https for all img src HTML.

This is the WordPress function I'm using to display images:

<img src="'.get_bloginfo("template_url").'/images/thumb-default.gif" />

This outputs an http img src - how can I convert that to https?

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up vote 8 down vote accepted

WordPress checks the return value of is_ssl() before creating URLs using get_bloginfo(). If the function returns true, it creates https URLs. If it returns false, it creates http URLs.

From the WordPress source ...

function is_ssl() {
    if ( isset($_SERVER['HTTPS']) ) {
        if ( 'on' == strtolower($_SERVER['HTTPS']) )
            return true;
        if ( '1' == $_SERVER['HTTPS'] )
            return true;
    } elseif ( isset($_SERVER['SERVER_PORT']) && ( '443' == $_SERVER['SERVER_PORT'] ) ) {
        return true;
    return false;

So ... if the request was made over https or if the request came in over port 443, then get_bloginfo() will return an https:// URL. In reality, if you're forcing https anyway, you should force all requests to port 80 (http) over to port 443 (https) ... but that's a server configuration issue, not a WordPress issue.

Alternatively, you could hook into a filter and replace http with https ...

Just use:

function replace_http( $original ) {
    // use preg_replace() to replace http:// with https://
    $output = preg_replace( "^http:", "https:", $original );
    return $output;

add_filter( 'template_url', 'replace_http' );
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Had you checked this when requesting page via http or https link? What does is_ssl() return for you?

I don't have SSL-capable stack to test at moment, but I am pretty sure that WP functions that put together links should be aware of SSL and output correct version.

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The page URL in my browser is https, but the img src links are http. – Ryan Apr 18 '11 at 18:11
@Ryan this is not normal behavior, I've checked and it should be https link generated in such case... see EAMann's answer for extended details. – Rarst Apr 18 '11 at 18:13

Using the home_url() function will automatically detect ssl and change protocols.

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To add to the fix proposed by EAMann I have made some changes for version 3.5:

add_filter( 'template_directory_uri', function( $original ) {
    $output = preg_replace( "/^http:/i", "https:", $original );
    return $output;

I'd like to note that this redirect thing may be very helpful, but my server redirects SSL to another port and is not detected by WP so for me it's an annoyance.

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Are you referring to WordPress 3.5? Is so, that's great! – Ryan Apr 17 '13 at 17:23

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