Variants, CSS, JS, and PHP

Every A/B test has at least a Control and a Treatment. Control is the current experience. Treatments are the changes you want to measure. Free is limited to those two arms. Pro can add more variants (A/B/n) and set each arm’s traffic weight.

What belongs on each arm

Control usually has empty snippets so the visitor sees the live theme unchanged.
Treatment holds the CSS, JS, and/or PHP you are testing. The list badges show which languages a test (and each arm) actually uses.
Weights (Pro) control how assigned traffic is split among arms after holdout is removed. Equal weights (for example 50 / 50) are the usual starting point.

When snippets run

Assignment happens on the HTML request (wp, priority 0), before the page is painted:

CSS is printed in wp_head for the assigned variant.
JS is printed in wp_footer.
PHP (Pro) is evaluated on the same request after a syntax check. Invalid snippets are skipped and logged; they are not executed.

That server-side timing is why targeted URLs cannot be fully page-cached while a test is running — the first HTML response has to include the correct variant. See Page Cache and Running Tests.

Writing CSS and JS

Scope selectors tightly (a specific class or page wrapper) so you do not restyle the whole store by accident.
JS runs in the footer. Wait for DOM ready if you query elements.
Do not put secrets, API keys, or unsanitized user input in snippets.
Prefer small, reversible changes. Large theme rewrites are better as a staging deploy plus a feature flag.

PHP snippets (Pro)

PHP is for changes that must exist in the first HTML: filters, template tweaks, or enqueueing assets. The snippet is admin-owned code, same trust model as a Code Snippets plugin. Do not include opening <?php tags. Runtime errors are caught and written to the experiments log; they do not fatally break the storefront.

For anything more than a few lines, put the logic in your theme or a small plugin and gate it:

if ( function_exists( 'wpdai_in_experiment' ) && wpdai_in_experiment( 'pricing-headline', 'treatment' ) ) {
    // Treatment-only markup or filters.
}

Theme and plugin branching