diff --git a/PgCache_Environment.php b/PgCache_Environment.php
index 52741cd4c..1af625595 100644
--- a/PgCache_Environment.php
+++ b/PgCache_Environment.php
@@ -482,8 +482,30 @@ private function wp_config_add_directive() {
 		}
 
 		$new_config_data = $this->wp_config_remove_from_content( $config_data );
+
+		/*
+		 * Insert the WP_CACHE directive immediately after the opening PHP tag. When a
+		 * strict_types declaration follows the tag, insert after that declaration instead,
+		 * so the directive never displaces strict_types from being the first statement in
+		 * the file (which is a fatal error). Comments between the tag and the declaration,
+		 * and additional directives inside it (e.g. "declare(strict_types=1, ticks=1)"),
+		 * are all tolerated.
+		 */
 		$new_config_data = preg_replace(
-			'~<\?(php)?~',
+			'~
+				<\?(?:php)?                    # Opening PHP tag: "<?php" or "<?".
+				(?:                            # Optionally consume through a strict_types declare.
+					(?:                        # Whitespace and comments preceding the declare:
+						\s                     #   whitespace,
+						| /\*.*?\*/            #   a block comment,
+						| //[^\r\n]*           #   a line comment, or
+						| \#[^\r\n]*           #   a hash comment.
+					)*
+					declare\s*\(               # "declare("
+						[^)]*strict_types[^)]* #   a directive list containing "strict_types"
+					\)\s*;                     # ");"
+				)?
+			~six',
 			"\\0\r\n" . $this->wp_config_addon(),
 			$new_config_data,
 			1
diff --git a/tests/test-wp-config-directive.php b/tests/test-wp-config-directive.php
new file mode 100644
index 000000000..75b9c0e61
--- /dev/null
+++ b/tests/test-wp-config-directive.php
@@ -0,0 +1,185 @@
+<?php
+/**
+ * Standalone regression test for PgCache_Environment::wp_config_add_directive() insertion.
+ *
+ * Verifies that the WP_CACHE directive is inserted immediately after the opening PHP
+ * tag, and — when a `declare(strict_types=1);` statement follows the tag — *after* that
+ * declaration, so it never displaces strict_types from being the first statement in the
+ * file (which would be a fatal error).
+ *
+ * Run with: php tests/test-wp-config-directive.php
+ *
+ * @package W3TC\Tests
+ * @since   2.10.0
+ */
+
+if ( realpath( __FILE__ ) !== realpath( $_SERVER['SCRIPT_FILENAME'] ?? '' ) ) {
+	return;
+}
+
+$wpcd_pass = 0;
+$wpcd_fail = 0;
+
+/**
+ * The directive block appended by {@see W3TC\PgCache_Environment::wp_config_addon()}.
+ *
+ * @var string
+ */
+$wpcd_addon = "/** Enable W3 Total Cache */\r\ndefine('WP_CACHE', true); // Added by W3 Total Cache\r\n";
+
+/**
+ * Mirrors the insertion performed by {@see W3TC\PgCache_Environment::wp_config_add_directive()}.
+ *
+ * @param string $config_data Raw wp-config.php contents.
+ * @param string $addon       Directive block to insert.
+ * @return string
+ */
+function wpcd_insert_directive( $config_data, $addon ) {
+	return preg_replace(
+		'~
+			<\?(?:php)?                    # Opening PHP tag: "<?php" or "<?".
+			(?:                            # Optionally consume through a strict_types declare.
+				(?:                        # Whitespace and comments preceding the declare:
+					\s                     #   whitespace,
+					| /\*.*?\*/            #   a block comment,
+					| //[^\r\n]*           #   a line comment, or
+					| \#[^\r\n]*           #   a hash comment.
+				)*
+				declare\s*\(               # "declare("
+					[^)]*strict_types[^)]* #   a directive list containing "strict_types"
+				\)\s*;                     # ");"
+			)?
+		~six',
+		"\\0\r\n" . $addon,
+		$config_data,
+		1
+	);
+}
+
+/**
+ * Exact-match assertion.
+ *
+ * @param string $label    Case label.
+ * @param string $expected Expected output.
+ * @param string $actual   Actual output.
+ */
+function wpcd_assert_equals( $label, $expected, $actual ) {
+	global $wpcd_pass, $wpcd_fail;
+
+	if ( $expected === $actual ) {
+		++$wpcd_pass;
+		echo "  \033[32m✔\033[0m  {$label}\n";
+		return;
+	}
+
+	++$wpcd_fail;
+	echo "  \033[31m✘\033[0m  {$label}\n";
+	echo '        expected: ' . var_export( $expected, true ) . "\n";
+	echo '        got:      ' . var_export( $actual, true ) . "\n";
+}
+
+/**
+ * Assertion that $needle appears before $before in $haystack (both must be present).
+ *
+ * @param string $label    Case label.
+ * @param string $haystack Output to inspect.
+ * @param string $needle   Substring expected first.
+ * @param string $before   Substring expected after $needle.
+ */
+function wpcd_assert_precedes( $label, $haystack, $needle, $before ) {
+	global $wpcd_pass, $wpcd_fail;
+
+	$pos_needle = strpos( $haystack, $needle );
+	$pos_before = strpos( $haystack, $before );
+	$ok         = false !== $pos_needle && false !== $pos_before && $pos_needle < $pos_before;
+
+	if ( $ok ) {
+		++$wpcd_pass;
+		echo "  \033[32m✔\033[0m  {$label}\n";
+		return;
+	}
+
+	++$wpcd_fail;
+	echo "  \033[31m✘\033[0m  {$label} (needle@" . var_export( $pos_needle, true ) . ', before@' . var_export( $pos_before, true ) . ")\n";
+}
+
+echo "\n=== wp-config.php WP_CACHE directive insertion ===\n\n";
+
+// 1. Standard long open tag: directive inserted immediately after `<?php` (backward compat).
+$input    = "<?php\r\n\r\n\$table_prefix = 'wp_';\r\n";
+$expected = '<?php' . "\r\n" . $wpcd_addon . "\r\n\r\n\$table_prefix = 'wp_';\r\n";
+wpcd_assert_equals( 'standard <?php tag: directive follows opening tag', $expected, wpcd_insert_directive( $input, $wpcd_addon ) );
+
+// 2. strict_types on its own line: directive inserted AFTER the declare statement.
+$input    = "<?php\r\n\r\ndeclare(strict_types=1);\r\n\r\n\$table_prefix = 'wp_';\r\n";
+$expected = "<?php\r\n\r\ndeclare(strict_types=1);" . "\r\n" . $wpcd_addon . "\r\n\r\n\$table_prefix = 'wp_';\r\n";
+$actual   = wpcd_insert_directive( $input, $wpcd_addon );
+wpcd_assert_equals( 'declare(strict_types=1) on next line: directive follows declare', $expected, $actual );
+wpcd_assert_precedes( 'strict_types stays first statement (before WP_CACHE)', $actual, 'strict_types', "define('WP_CACHE'" );
+
+// 3. strict_types on the same line as the open tag.
+$input    = "<?php declare(strict_types=1);\r\n\$table_prefix = 'wp_';\r\n";
+$expected = '<?php declare(strict_types=1);' . "\r\n" . $wpcd_addon . "\r\n\$table_prefix = 'wp_';\r\n";
+$actual   = wpcd_insert_directive( $input, $wpcd_addon );
+wpcd_assert_equals( 'declare(strict_types=1) on same line: directive follows declare', $expected, $actual );
+wpcd_assert_precedes( 'same-line strict_types stays first statement', $actual, 'strict_types', "define('WP_CACHE'" );
+
+// 4. Short open tag.
+$input    = "<?\r\n\$x = 1;\r\n";
+$expected = '<?' . "\r\n" . $wpcd_addon . "\r\n\$x = 1;\r\n";
+wpcd_assert_equals( 'short open tag <?: directive follows opening tag', $expected, wpcd_insert_directive( $input, $wpcd_addon ) );
+
+// 5. Case-insensitive keyword and internal whitespace inside declare().
+$input    = "<?php\r\nDECLARE( strict_types = 1 );\r\n\$x = 1;\r\n";
+$expected = "<?php\r\nDECLARE( strict_types = 1 );" . "\r\n" . $wpcd_addon . "\r\n\$x = 1;\r\n";
+$actual   = wpcd_insert_directive( $input, $wpcd_addon );
+wpcd_assert_equals( 'uppercase DECLARE with inner spaces: directive follows declare', $expected, $actual );
+wpcd_assert_precedes( 'spaced/uppercase strict_types stays first statement', $actual, 'strict_types', "define('WP_CACHE'" );
+
+// 6. Comment (docblock) between the open tag and the strict_types declare.
+$input    = "<?php\r\n/**\r\n * The base configuration.\r\n */\r\ndeclare(strict_types=1);\r\n\$x = 1;\r\n";
+$expected = "<?php\r\n/**\r\n * The base configuration.\r\n */\r\ndeclare(strict_types=1);" . "\r\n" . $wpcd_addon . "\r\n\$x = 1;\r\n";
+$actual   = wpcd_insert_directive( $input, $wpcd_addon );
+wpcd_assert_equals( 'docblock before declare: directive follows declare', $expected, $actual );
+wpcd_assert_precedes( 'docblock case: strict_types stays first statement', $actual, 'strict_types', "define('WP_CACHE'" );
+
+// 7. Line/hash comments between the open tag and the declare.
+$input    = "<?php\r\n// a note\r\n# another\r\ndeclare(strict_types=1);\r\n\$x = 1;\r\n";
+$expected = "<?php\r\n// a note\r\n# another\r\ndeclare(strict_types=1);" . "\r\n" . $wpcd_addon . "\r\n\$x = 1;\r\n";
+$actual   = wpcd_insert_directive( $input, $wpcd_addon );
+wpcd_assert_equals( 'line/hash comments before declare: directive follows declare', $expected, $actual );
+wpcd_assert_precedes( 'comment case: strict_types stays first statement', $actual, 'strict_types', "define('WP_CACHE'" );
+
+// 8. Multi-directive declare, strict_types first.
+$input    = "<?php\r\ndeclare(strict_types=1, ticks=1);\r\n\$x = 1;\r\n";
+$expected = "<?php\r\ndeclare(strict_types=1, ticks=1);" . "\r\n" . $wpcd_addon . "\r\n\$x = 1;\r\n";
+$actual   = wpcd_insert_directive( $input, $wpcd_addon );
+wpcd_assert_equals( 'declare(strict_types=1, ticks=1): directive follows declare', $expected, $actual );
+wpcd_assert_precedes( 'multi-directive case: strict_types stays first statement', $actual, 'strict_types', "define('WP_CACHE'" );
+
+// 9. Multi-directive declare, strict_types last.
+$input    = "<?php\r\ndeclare(ticks=1, strict_types=1);\r\n\$x = 1;\r\n";
+$expected = "<?php\r\ndeclare(ticks=1, strict_types=1);" . "\r\n" . $wpcd_addon . "\r\n\$x = 1;\r\n";
+$actual   = wpcd_insert_directive( $input, $wpcd_addon );
+wpcd_assert_equals( 'declare(ticks=1, strict_types=1): directive follows declare', $expected, $actual );
+wpcd_assert_precedes( 'strict_types-last case: strict_types stays first statement', $actual, 'strict_types', "define('WP_CACHE'" );
+
+// 10. A non-strict_types declare must NOT be treated as strict_types: directive goes after the tag,
+// before the declare (ticks has no "first statement" restriction).
+$input    = "<?php\r\ndeclare(ticks=1);\r\n\$x = 1;\r\n";
+$expected = '<?php' . "\r\n" . $wpcd_addon . "\r\ndeclare(ticks=1);\r\n\$x = 1;\r\n";
+wpcd_assert_equals( 'declare(ticks=1) only: directive follows the opening tag', $expected, wpcd_insert_directive( $input, $wpcd_addon ) );
+
+// 11. A standard WordPress docblock (no declare) must not be skipped: directive follows the tag.
+$input    = "<?php\r\n/**\r\n * The base configuration for WordPress.\r\n */\r\ndefine('DB_NAME', 'wp');\r\n";
+$expected = '<?php' . "\r\n" . $wpcd_addon . "\r\n/**\r\n * The base configuration for WordPress.\r\n */\r\ndefine('DB_NAME', 'wp');\r\n";
+wpcd_assert_equals( 'docblock but no declare: directive follows the opening tag', $expected, wpcd_insert_directive( $input, $wpcd_addon ) );
+
+// 12. Only one directive is inserted even when the file body mentions the open tag again.
+$input  = "<?php\r\n\$a = '<?php';\r\n";
+$actual = wpcd_insert_directive( $input, $wpcd_addon );
+wpcd_assert_equals( 'single insertion only', 1, substr_count( $actual, "define('WP_CACHE'" ) );
+
+echo "\n  Total: " . ( $wpcd_pass + $wpcd_fail ) . "  Passed: \033[32m{$wpcd_pass}\033[0m  Failed: \033[31m{$wpcd_fail}\033[0m\n\n";
+
+exit( $wpcd_fail > 0 ? 1 : 0 );