Fatal error: Uncaught TypeError: count(): Argument #1 ($value) must be of type Countable|array, … given

Since PHP 8.0, count() throws a TypeError when it receives something other than an array or a Countable object. PHP 7 only showed a warning, so older plugins and themes that count null or a string stop the site after a PHP upgrade.

Common causes

References

1. count() (PHP.net)
2. Backward incompatible changes in PHP 8.0 (PHP.net)

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1. Initial assessment: Site URL entry
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3. Diagnosis: Item-by-item checks
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5. Write-back or delivery: Only after verification passes