三元运算符中的评估顺序

Question

I have two snippets of php:

function prefix_shortcode_statement_conditional( $atts, $content = null ) {
    return '<li class="some-class">' . $content == null ? '' : $content . '</li>';
}

function prefix_shortcode_statement( $atts, $content = null ) {
    return '<li class="some-class">' . $content . '</li>';
}

The first snippet has a conditional statement to return an empty string as the <li> content, while the second one merely returns the content wrapped in <li>.

prefix_shortcode_statement_conditional output

Lorem Ipsum ...

prefix_shortcode_statement output

<li class="some-class">
    Lorem Ipsum ... 
</li>

As you can see, only the function without the conditional statement properly generates the <li> element. Why does one snippet generate the code, but not the other?

---

Wrapping the conditional in parentheses resolves the issue, but fails to help me understand why the issue arose in the first place.

return '<li class="some-class">' . ($content == null ? '' : $content) . '</li>';

As far as I can tell, php should interpret ending </li> as part of the conditional statement, but fails to. Instead of adding $content . '</li>' to the output, it instead just returns $content and at some level (browser, wordpress, etc), the first <li> is removed from the outputted html.

Interestingly, wrapping the $content and the </li> also fails to display the <li>...</li>.

return '<li class="mep-testimonial-li">' . $content == null ? '' : ($content . '</li>');

I would expect to only see this issue when $content is null, so the referenced </li> would be omitted and the formatting would shift.

Answer 1

In PHP, as in other languages, concatenation have a higher precedence than comparison, so:

return '<li class="some-class">' . $content == null ? '' : $content . '</li>';

Will always print $content . '</li>', since '<li class="some-class">' . $content will never be falsy ou null.
This may lead to invalid markup and not be rendered correctly in the browser.

Always wrap your ternaries in parenthesis.

return '<li class="some-class">' . ($content == null ? '' : $content) . '</li>';