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<?php function doFoo(){ echo 'hi!'; } $callable = 'doFoo'; var_dump( is_callable( $callable ) ); var_dump( is_string( $callable ) ); function doBar( callable $cb ){ $cb(); } doBar( $callable );
Output for 5.4.0 - 5.4.45, 5.5.0 - 5.5.35, 5.6.0 - 5.6.28, 7.0.0 - 7.0.20, 7.1.0 - 7.1.33, 7.2.0 - 7.2.33, 7.3.0 - 7.3.33, 7.4.0 - 7.4.33, 8.0.0 - 8.0.30, 8.1.0 - 8.1.28, 8.2.0 - 8.2.18, 8.3.0 - 8.3.6
bool(true) bool(true) hi!
Output for 5.2.3 - 5.2.17, 5.3.0 - 5.3.29
bool(true) bool(true) Catchable fatal error: Argument 1 passed to doBar() must be an instance of callable, string given, called in /in/u9cST on line 13 and defined in /in/u9cST on line 9
Process exited with code 255.
Output for 5.2.0 - 5.2.2
bool(true) bool(true) <br /> <b>Catchable fatal error</b>: Argument 1 passed to doBar() must be an instance of callable, string given, called in /in/u9cST on line 13 and defined in <b>/in/u9cST</b> on line <b>9</b><br />
Process exited with code 255.
Output for 5.1.0 - 5.1.6
bool(true) bool(true) <br /> <b>Fatal error</b>: Argument 1 passed to doBar() must be an object of class callable, called in /in/u9cST on line 13 and defined in <b>/in/u9cST</b> on line <b>9</b><br />
Process exited with code 255.
Output for 5.0.0 - 5.0.5
bool(true) bool(true) <br /> <b>Fatal error</b>: Argument 1 must be an object of class callable in <b>/in/u9cST</b> on line <b>9</b><br />
Process exited with code 255.
Output for 4.4.2 - 4.4.9
<br /> <b>Parse error</b>: syntax error, unexpected T_STRING, expecting ')' in <b>/in/u9cST</b> on line <b>9</b><br />
Process exited with code 255.
Output for 4.3.0 - 4.3.1, 4.3.5 - 4.3.11, 4.4.0 - 4.4.1
<br /> <b>Parse error</b>: parse error, unexpected T_STRING, expecting ')' in <b>/in/u9cST</b> on line <b>9</b><br />
Process exited with code 255.
Output for 4.3.2 - 4.3.4
<br /> <b>Parse error</b>: parse error, expecting `')'' in <b>/in/u9cST</b> on line <b>9</b><br />
Process exited with code 255.

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