Guard select field update_value against nested-array values

[cbravobernal]

Description

ACF_Field_Select::update_value() stringifies the submitted value via array_map( 'strval', $value ) so that each value can be matched by SQL LIKE queries. This assumes every element of $value is a scalar.

A multi-value select can receive a value where an element is itself an array — for example via a nested POST such as acf[field_key][0][]=x. Passing such an element through strval() raises an Array to string conversion warning. This should be handled rather than emitting the diagnostic.

This PR makes the smallest sensible guard: stringify only scalar elements and coerce any non-scalar element to an empty string. Normal scalar arrays of values are unaffected.

// Before
$value = array_map( 'strval', $value );

// After
$value = array_map(
    static function ( $v ) {
        return is_scalar( $v ) ? strval( $v ) : '';
    },
    $value
);

The checkbox field delegates to this method (acf_get_field_type( 'select' )->update_value( ... )), so it is covered by the same guard. The radio field has its own update_value() that operates on a scalar value and does not use this code path, so it is unaffected.

This file is upstream-derived; the same guard applies upstream.

Verification

• New regression test Test_ACF_Field_Select::test_update_value_nested_array fails against the unpatched code (PHPUnit is configured with convertWarningsToExceptions, so the Array to string conversion warning surfaces as a failure) and passes with the fix.
• composer test:php -- --filter 'Field_Select' — green (17 tests).
• Full composer test:php — green (2265 tests).
• composer test:phpstan — clean.
• phpcs on the changed files introduces no new issues (the select field's pre-existing legacy warnings are unchanged; the test file is clean).

Closes

Use of AI Tools

This PR was authored with Claude Code under human direction. All changes were reviewed by a human, who takes responsibility for the contribution.

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